safefree use: fix compiler warning

include the prototype header

lib/http_negotiate.c

@@ -37,6 +37,7 @@
 #include "curl_base64.h"
 #include "http_negotiate.h"
 #include "curl_memory.h"
+#include "url.h"
 
 #ifdef HAVE_SPNEGO
 #  include <spnegohelp.h>