mesa: tweak output of _mesa_write_shader_to_file()

Enclose GPU code in comments to the file can be re-fed back into GLSL compiler.
diff --git a/src/mesa/shader/prog_print.c b/src/mesa/shader/prog_print.c
index fa1d361..8b0f16e 100644
--- a/src/mesa/shader/prog_print.c
+++ b/src/mesa/shader/prog_print.c
@@ -902,7 +902,9 @@
    }
    else {
       fprintf(f, "/* GPU code */\n");
+      fprintf(f, "/*\n");
       _mesa_fprint_program_opt(f, shader->Program, PROG_PRINT_DEBUG, GL_TRUE);
+      fprintf(f, "*/\n");
    }
 
    fclose(f);