handshake: add ERP cache object to handshake

Keeping the ERP cache on the handshake object allows station.c to
handle all the ERP details and encapsulate them into a handshake.
FILS can then use the ERP cache right from the handshake rather
than getting it itself.

src/handshake.h

@@ -118,6 +118,7 @@ struct handshake_state {
 	uint8_t gtk[32];
 	uint8_t gtk_rsc[6];
 	unsigned int gtk_index;
+	struct erp_cache_entry *erp_cache;
 	void *user_data;
 
 	void (*free)(struct handshake_state *s);