tests/gem_pwrite_snooped: disable const cast warning

Disable -Wcast-qual temporarily to allow memchr_inv to return non-const
data (similar to memchr), without causing a compiler warning.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thomas Wood <thomas.wood@intel.com>

tests/gem_pwrite_snooped.c

@@ -83,7 +83,10 @@ static void *memchr_inv(const void *s, int c, size_t n)
 
 	while (n--) {
 		if (*us != uc)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-qual"
 			return (void *) us;
+#pragma GCC diagnostic pop
 		us++;
 	}