meson: Build gallium dri state tracker

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric at anholt.net>
diff --git a/meson.build b/meson.build
index fecb5db..4cde8c7 100644
--- a/meson.build
+++ b/meson.build
@@ -92,6 +92,7 @@
 
 with_gallium = false
 with_gallium_radeonsi = false
+with_gallium_softpipe = false
 _drivers = get_option('gallium-drivers')
 if _drivers != ''
   _split = _drivers.split(',')