gnused: Fix cross build after upgrade to 4.3.0

Fixes:
````
  GEN      doc/sed.1
help2man: can't get `--help' info from sed/sed
make[2]: *** [Makefile:5775: doc/sed.1] Error 126
make[2]: Leaving directory '/tmp/nix-build-gnused-4.3-arm-linux-gnueabihf.drv-0/sed-4.3'
make[1]: *** [Makefile:3024: all-recursive] Error 1
make[1]: Leaving directory '/tmp/nix-build-gnused-4.3-arm-linux-gnueabihf.drv-0/sed-4.3'
make: *** [Makefile:2156: all] Error 2
````

http://hydra.nixos.org/build/46051086/nixlog/11/raw