glmark2: narrow down libgl requirements

Mesa3d DRI drivers can be enabled without support for libgl (for
example without x11). So make glmark2 gl (x11) support depend on
BR2_PACKAGE_HAS_LIBGL being present as well.
It doesn't exactly mean mesa3d providing libgl, but since we can't have
more than one libgl provider at the same time it's equivalent.
This is fallout from f1894ec9 which
enabled DRI drivers for non-xorg builds. Fixes:
http://autobuild.buildroot.net/results/174/1743ee5a340e5f4b1f8519e0b11eb196caa4f30e/



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