glsl: remove unneeded extern "C" {} bracketing around Mesa includes

The two headers already have the right extern "C" annotations.

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

src/compiler/glsl/serialize.cpp

@@ -33,13 +33,11 @@
 #include "compiler/shader_info.h"
 #include "ir_uniform.h"
 #include "main/mtypes.h"
+#include "main/shaderobj.h"
+#include "program/program.h"
 #include "string_to_uint_map.h"
 #include "util/bitscan.h"
 
-extern "C" {
-#include "main/shaderobj.h"
-#include "program/program.h"
-}
 
 static void
 write_subroutines(struct blob *metadata, struct gl_shader_program *prog)