glsl: include util/bitscan.h in serialize.cpp

Instead of relying on indirect inclusion of the header.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/compiler/glsl/serialize.cpp

@@ -34,6 +34,7 @@
 #include "ir_uniform.h"
 #include "main/mtypes.h"
 #include "string_to_uint_map.h"
+#include "util/bitscan.h"
 
 extern "C" {
 #include "main/shaderobj.h"