Makefile: install gtk message catalogs if CONFIG_GTK=y too, not only =m

Fixes 722cd7496474cebb2218f21e038592fad8603365

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180416093719.2543-1-mjt@msgid.tls.msk.ru
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Makefile

@@ -856,7 +856,7 @@ ifneq ($(BLOBS),)
 		$(INSTALL_DATA) $(SRC_PATH)/pc-bios/$$x "$(DESTDIR)$(qemu_datadir)"; \
 	done
 endif
-ifeq ($(CONFIG_GTK),m)
+ifdef CONFIG_GTK
 	$(MAKE) -C po $@
 endif
 	$(INSTALL_DIR) "$(DESTDIR)$(qemu_datadir)/keymaps"