make: switch language file build to be gtk module aware

Now that gtk support builds as a module, CONFIG_GTK changed from
y to m. Adjust Makefile correspondingly.
Signed-off-by: NBruce Rogers <brogers@suse.com>
Message-id: 20180307155517.32570-1-brogers@suse.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

Makefile

@@ -851,7 +851,7 @@ ifneq ($(BLOBS),)
 		$(INSTALL_DATA) $(SRC_PATH)/pc-bios/$$x "$(DESTDIR)$(qemu_datadir)"; \
 	done
 endif
-ifeq ($(CONFIG_GTK),y)
+ifeq ($(CONFIG_GTK),m)
 	$(MAKE) -C po $@
 endif
 	$(INSTALL_DIR) "$(DESTDIR)$(qemu_datadir)/keymaps"