meson: Add support for the vc5 driver.

v2: Default vc5 to off, since it requires the simulator currently.  Add
    missing dep on the XML generation from libbroadcom_vc5.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com> (v1)
diff --git a/meson.build b/meson.build
index 9967df0..5c42102 100644
--- a/meson.build
+++ b/meson.build
@@ -96,6 +96,7 @@
 with_gallium_nouveau = false
 with_gallium_softpipe = false
 with_gallium_vc4 = false
+with_gallium_vc5 = false
 _drivers = get_option('gallium-drivers')
 if _drivers != ''
   _split = _drivers.split(',')
@@ -104,6 +105,7 @@
   with_gallium_nouveau = _split.contains('nouveau')
   with_gallium_softpipe = _split.contains('swrast')
   with_gallium_vc4 = _split.contains('vc4')
+  with_gallium_vc5 = _split.contains('vc5')
   with_gallium = true
   with_dri = true
 endif