package/libgtk3: fix cups backend

When explicitly specifying printing backends libgtk3 requires file
to be one of them on linux.

Fixes:
- output/build/libgtk3-3.24.37/modules/printbackends/meson.build:39:4: ERROR: Problem encountered: 'file' print backed needs to be enabled

Signed-off-by: Daniel Lang <dalang@gmx.at>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 9a0857d734c5fca74915b1cd85b92555fe15fb93)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libgtk3/libgtk3.mk

@@ -68,7 +68,7 @@ LIBGTK3_DEPENDENCIES += xlib_libXdamage
 endif
 
 ifeq ($(BR2_PACKAGE_CUPS),y)
-LIBGTK3_CONF_OPTS += -Dprint_backends=cups
+LIBGTK3_CONF_OPTS += -Dprint_backends=file,cups
 LIBGTK3_DEPENDENCIES += cups
 else
 LIBGTK3_CONF_OPTS += -Dprint_backends=auto