meson: Add dridriverdir variable to dri.pc.

Xorg (and possibly other things) depend on this variable to find the
path to DRI drivers.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/mesa/drivers/dri/meson.build

@@ -58,6 +58,7 @@ if dri_drivers != []
     filebase : 'dri',
     description : 'Direct Rendering Infrastructure',
     version : meson.project_version(),
+    variables : ['dridriverdir=${prefix}/' + dri_drivers_path],
     requires_private : ['libdrm >= 2.4.75'],  # FIXME: don't hardcode this
   )