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

Fixes 722cd749

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>