glsl: Add strings.h on non-MSC platforms

* IEEE Std 1003.1-2001 placed strcasecmp() in strings.h.
* ISO C99 doesn't mention strcase* in string.h
* On all platforms I could find, strcasecmp is in strings.h and string.h
  as a compatibility layer for software written pre-2001 POSIX
* Technically strcasecmp should be only in strings.h and the man
  pages back this up.
* Tested build on CentOS and Haiku

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/glsl_parser.yy

@@ -24,6 +24,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifndef _MSC_VER
+#include <strings.h>
+#endif
 #include <assert.h>
 
 #include "ast.h"