glsl: #include c99_math.h instead of core.h

We only need the M_LOG2E definition.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/glsl/lower_instructions.cpp

@@ -124,7 +124,7 @@
  * Converts double trunc, ceil, floor, round to fract
  */
 
-#include "main/core.h" /* for M_LOG2E */
+#include "c99_math.h"
 #include "program/prog_instruction.h" /* for swizzle */
 #include "glsl_types.h"
 #include "ir.h"