package/ushare: fix build without msgfmt

Fix the following build failure without /usr/bin/msgfmt raised since the
addition of ushare in commit 74097fd6:

make[3]: Entering directory `/home/buildroot/autobuild/run/instance-3/output-1/build/ushare-2.1/po'
/usr/bin/msgfmt -c --statistics -o fr.gmo fr.po
make[3]: /usr/bin/msgfmt: Command not found

To fix this build failure, set GMSGFMT to $(HOST_DIR)/bin/msgfmt and
don't build po files if NLS is disabled

Fixes:
 - http://autobuild.buildroot.org/results/9f6b5b8f38386135bacd2d8f6e97c1fea77bbe69



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>