git: fix gitweb.cgi breakage due to missing CGI.pm

CGI.pm was removed from Perl core in v5.22, breaking the gitweb.cgi
script and thus NixOS' services.lighttpd.gitweb. Nixpkgs switched to
Perl 5.22 with a85ba820 ("perl: Make 5.22 the default").

Fix it by bringing in the Perl CGI module (and dependency HTMLParser).
The closure size of "nix-build -A git" increase by 1 MiB, to 201 MiB.