package/mesa3d: provides libgl not only with DRI drivers

This patch is a follow-up for
https://github.com/buildroot/buildroot/commit/999d4a263118ab8581190d3e3c8bdc471896628f

libGL.so and support files are not only available with DRI drivers but
also with Gallium drivers, in fact mesa3d can be built without any
driver enabled - unless X.org is disabled it will provide libGL.so.

Tested using this defconfig:
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_KODI=y
BR2_PACKAGE_KODI_VISUALISATION_WAVEFORHUE=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_XORG7=y

Fixes
http://autobuild.buildroot.net/results/d97/d97a80e6de066a7dea08b284eab24fc44b11d661/



kodi-visualisation-waveforhue detects OpenGL installed by mesa3d but the
kodi package itself does not select libglew before because mesa3d was
not listed as a provider for libgl in the buildroot build system.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>