gallium: Add missing file for scons build.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index a77c3de..678e4ad 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -161,6 +161,7 @@
 	'state_tracker/st_extensions.c',
 	'state_tracker/st_format.c',
 	'state_tracker/st_framebuffer.c',
+	'state_tracker/st_gen_mipmap.c',
 	'state_tracker/st_mesa_to_tgsi.c',
 	'state_tracker/st_program.c',
 	'state_tracker/st_texture.c',