virglrenderer: explicitly depend on libgbm

virtual/gl may not necessarily be mesa, and virgl needs
specifically the gbm part of mesa.

There's also hope nvidia will support gbm somehow, someday.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>