gettext-full: add gmsgfmt symlink in host install

gettext-full: add gmsgfmt symlink in host install

Some configure scripts look for msgfmt and gmsgfmt. As we don't install
the latter, configure might pick up one from staging_dir/hostpkg, and
the other from the host:

checking for msgfmt... /home/stijn/Development/OpenWrt/openwrt/staging_dir/hostpkg/bin/msgfmt
checking for gmsgfmt... /usr/bin/gmsgfmt

This could potentially lead to hard to debug undefined behaviour.
Install a symlink in the host install phase to avoid this.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>