winsys/sw/null: Convert to automake
diff --git a/configure.ac b/configure.ac
index 5771af3..032b215 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2035,6 +2035,7 @@
 		src/gallium/winsys/sw/Makefile
 		src/gallium/winsys/sw/dri/Makefile
 		src/gallium/winsys/sw/fbdev/Makefile
+		src/gallium/winsys/sw/null/Makefile
 		src/gbm/Makefile
 		src/gbm/main/gbm.pc
 		src/glsl/Makefile