modem-manager-gui: Fix build

Attempt to fix the following issue:

	Running custom install script '/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/bin/meson --internal yelphelper install --subdir=help --id=modem-manager-gui --installdir=share/help --sources=about.page@@contrib-code.page@@contrib-translations.page@@index.page@@license.page@@report-bugs.page@@usage-config.page@@usage-contacts.page@@usage-getinfo.page@@usage-modem.page@@usage-netsearch.page@@usage-sms.page@@usage-traffic.page@@usage-ussd.page --symlinks=true --media=figures/contacts-window.png@@figures/modem-manager-gui-logo.png@@figures/network-info.png@@figures/scan-window.png@@figures/sms-window.png@@figures/traffic-window.png@@figures/ussd-window.png@@figures/startup-window.png'
	Traceback (most recent call last):
	  File "/nix/store/0jg6lgd7i6c3y4jjms8j0b0cigij94hn-itstool-2.0.6/bin/itstool", line 1598, in <module>
	    doc.merge_translations(translations, opts.lang, strict=opts.strict)
	  File "/nix/store/0jg6lgd7i6c3y4jjms8j0b0cigij94hn-itstool-2.0.6/bin/itstool", line 984, in merge_translations
	    lcpar = lcpar.parent
	  File "/nix/store/cp1g6m360dky1baz3x5q5gm92basj7rv-python3-3.7.7-env/lib/python3.7/site-packages/libxml2.py", line 296, in get_parent
	    return nodeWrap(ret)
	  File "/nix/store/cp1g6m360dky1baz3x5q5gm92basj7rv-python3-3.7.7-env/lib/python3.7/site-packages/libxml2.py", line 580, in nodeWrap
	    if name[0:8] == "document":
	TypeError: 'NoneType' object is not subscriptable
	[...]
	Installing /build/hg-archive/build/help/uk/usage-ussd.page to /nix/store/8miw71zkfjqd8z3v5zzp8ybv6pxmc33y-modem-manager-gui-0.0.19.1/share/help/uk/modem-manager-gui/usage-ussd.page
	Traceback (most recent call last):
	  File "/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/bin/meson", line 9, in <module>
	    sys.exit(main())
	  File "/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/lib/python3.7/site-packages/mesonbuild/mesonmain.py", line 226, in main
	    return run(sys.argv[1:], launcher)
	  File "/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/lib/python3.7/site-packages/mesonbuild/mesonmain.py", line 215, in run
	    return run_script_command(args[1], args[2:])
	  File "/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/lib/python3.7/site-packages/mesonbuild/mesonmain.py", line 163, in run_script_command
	    return module.run(script_args)
	  File "/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/lib/python3.7/site-packages/mesonbuild/scripts/yelphelper.py", line 132, in run
	    destdir, options.project_id, options.symlinks)
	  File "/nix/store/9z5fg8ia4lr10nqglk3jlwyd7xcqadzk-meson-0.53.2/lib/python3.7/site-packages/mesonbuild/scripts/yelphelper.py", line 71, in install_help
	    shutil.copyfile(infile, outfile)
	  File "/nix/store/vs4vj1yzqj1bkcqkf3b6sxm6jfy1gb4j-python3-3.7.7/lib/python3.7/shutil.py", line 120, in copyfile
	    with open(src, 'rb') as fsrc:
	FileNotFoundError: [Errno 2] No such file or directory: '/build/hg-archive/build/help/uk/usage-ussd.page'
	FAILED: meson-install