package/piglit: fix build of GLES tests

GLES tests needs libegl so:
- check for BR2_PACKAGE_HAS_LIBEGL to enable them
- add libegl to PIGLIT_DEPENDENCIES if needed

Fixes:
 - http://autobuild.buildroot.org/results/3c2b7fb1e54dd52a5fd9c1024b8258e02063bd2c



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>