package/mesa3d: vc4 backend is an EGL provider

The VC4 GPU does not support full GL, it only provides EGL.

Currently, it is possible to build the VC4 backend without EGL support,
but that does not make sense in the slighest.

So, forcibly enable EGL with VC4, like is done for etnaviv and virgl.

Reported-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>