glsl: Remove unused include from glsl_types.cpp

Found with IWYU. Compile-tested on my Ivy-bridge system.
Added comment about core.h being used for MAX2.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Thomas Helland <thomashelland90@gmail.com>

src/glsl/glsl_types.cpp

@@ -22,9 +22,7 @@
  */
 
 #include <stdio.h>
-#include <stdlib.h>
-#include "main/core.h" /* for Elements */
-#include "glsl_symbol_table.h"
+#include "main/core.h" /* for Elements, MAX2 */
 #include "glsl_parser_extras.h"
 #include "glsl_types.h"
 extern "C" {