opencv: gtk support doesn't work with directfb

This was dropped on 4b3d949b when
libgkt2 was updated. Fixes:
http://autobuild.buildroot.net/results/817/8173b7fd3d6af79a7f2c04bf36059a4429127516/



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