Cast away const in call to strstr()

My Linux man page says the arguments to this function should be const,
but the compiler refuses to compile it when so.  Easiest to just cast
them away; the function isn't going to change them.

Author: khwilliamson

util.h

@@ -252,7 +252,7 @@ returning NULL if not found.  The terminating NUL bytes are not compared.
 */
 
 
-#define instr(haystack, needle) strstr(haystack, needle)
+#define instr(haystack, needle) strstr((char *) haystack, (char *) needle)
 
 #ifdef HAS_MEMMEM
 #   define ninstr(big, bigend, little, lend)                                \