tests: adjust type for new compiler wanring

Preserve the const-ness of the string constants as identified by the newer
compiler.

Author: compnerd

tests/dispatch_context_for_key.c

@@ -31,7 +31,7 @@
 
 #if DISPATCH_API_VERSION >= 20100825 && DISPATCH_API_VERSION != 20101110
 
-static char *ctxts[] = {"ctxt for app", "ctxt for key 1",
+static const char *ctxts[] = {"ctxt for app", "ctxt for key 1",
 		"ctxt for key 2", "ctxt for key 1 bis", "ctxt for key 4"};
 volatile long ctxts_destroyed;
 static dispatch_group_t g;