gettext-full: always use shipped libcroco, unbreaks build if a different libcroco is installed on the host (#12539)

Based on patch from Joerg Hollmann <Joerg.Hollmann@t-online.de>

git-svn-id: svn://svn.openwrt.org/openwrt@37115 3c298f89-4303-0410-b956-a3cf2f4a3e73