cap-notify: Fix possible crash on 64-bit systems.

find_named_client() was called without a prototype and therefore the
pointer could be truncated.

modules/m_cap.c

@@ -43,6 +43,7 @@
 #include "s_user.h"
 #include "send.h"
 #include "s_conf.h"
+#include "hash.h"
 
 typedef int (*bqcmp)(const void *, const void *);