libgtk2: point to the proper cups-config

Otherwise it can pick up the distro cups-config if it's present, leading
to improper library usage/linkage. Fixes:
http://autobuild.buildroot.net/results/bf4/bf493977817a750427aae82e71a5257187bd7344/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libgtk2/libgtk2.mk

@@ -85,6 +85,7 @@ LIBGTK2_CONF_OPTS += --without-libtiff
 endif
 
 ifeq ($(BR2_PACKAGE_CUPS),y)
+LIBGTK2_CONF_OPTS += CUPS_CONFIG="$(STAGING_DIR)/usr/bin/cups-config"
 LIBGTK2_DEPENDENCIES += cups
 else
 LIBGTK2_CONF_OPTS += --disable-cups