travis: Enable vc4 in libdrm to satisfy vc4 test build dependency.
travis: Enable vc4 in libdrm to satisfy vc4 test build dependency.

Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Rhys Kidd <0b3b315d08353e2046d43a86d41e3fd1dff3d6ad@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler