brushlib tests: fix build failure sans enable_gegl
brushlib tests: fix build failure sans enable_gegl

Regular builds were requiring gegl-2.0, which isn't available
in Ubuntu 12.04.

b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint