issue #200: Rebuild .mo files when running "make"

The .mo files were only rebuilt with the command "make -C update-po",
but not from the default "make".

Make "update-gmo" a required dependency for the default target ("all")
in po/Makefile.

Fix issue #200.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>