po: generate .pot file with strings in alphabetical order

The .po files are stored with strings in alphabetical order instead of
source file location order, because this minimizes the diffs created
when code moves around within or between files.

By default msgmerge will honour the order of strings in the .pot file
when creating a .po file, so it is useful if we also create the .pot
file with desired ordering.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

po/Makefile.am

@@ -32,6 +32,7 @@ XGETTEXT_ARGS = \
 	--package-name="$(PACKAGE_NAME)" \
 	--package-version="$(PACKAGE_VERSION)" \
 	--msgid-bugs-address="$(MSGID_BUGS_ADDRESS)" \
+	--sort-output \
 	$(NULL)
 
 SED_PO_FIXUP_ARGS = \