meson: Add support for the vc4 driver.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
diff --git a/meson.build b/meson.build
index bc3bbf4..f538cf7 100644
--- a/meson.build
+++ b/meson.build
@@ -94,12 +94,14 @@
 with_gallium_radeonsi = false
 with_gallium_nouveau = false
 with_gallium_softpipe = false
+with_gallium_vc4 = false
 _drivers = get_option('gallium-drivers')
 if _drivers != ''
   _split = _drivers.split(',')
   with_gallium_radeonsi = _split.contains('radeonsi')
   with_gallium_nouveau = _split.contains('nouveau')
   with_gallium_softpipe = _split.contains('swrast')
+  with_gallium_vc4 = _split.contains('vc4')
   with_gallium = true
   with_dri = true
 endif