qt: fix build with opengl es

Fixes http://autobuild.buildroot.net/results/efe/efe26c54361094fb1e201da6915900cd9b24d596/



qt doesn't use pkg-config to figure out the needed compiler/linker flags
for egl, so pass them explicitly instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>