Fix wrong variable name in build file
Fix wrong variable name in build file

RevBy: Jan Arne Petersen
Use GTK3_DIR, not GTK2_DIR (probably a copy and paste bug).

Full, original commit at a66783cd8c9c6c596453a9592f0730e31620592e in maliit-framework

maliit/inputcontext-gtk,maliit/inputcontext-gtk