sphinx: Make the sphinx_hop struct public

We're about to create a param helper for sphinx hops and this struct seems
like the correct place to store the result.

common/sphinx.c

@@ -37,17 +37,6 @@ struct keyset {
 	u8 gamma[KEY_LEN];
 };
 
-/*
- * All the necessary information to generate a valid onion for this hop on a
- * sphinx path. The payload is preserialized in order since the onion
- * generation is payload agnostic. */
-struct sphinx_hop {
-	struct pubkey pubkey;
-	enum sphinx_payload_type type;
-	const u8 *payload;
-	u8 hmac[HMAC_SIZE];
-};
-
 /* Encapsulates the information about a given payment path for the the onion
  * routing algorithm.
  */

common/sphinx.h

@@ -75,6 +75,17 @@ enum sphinx_payload_type {
 	SPHINX_RAW_PAYLOAD = 255,
 };
 
+/*
+ * All the necessary information to generate a valid onion for this hop on a
+ * sphinx path. The payload is preserialized in order since the onion
+ * generation is payload agnostic. */
+struct sphinx_hop {
+	struct pubkey pubkey;
+	enum sphinx_payload_type type;
+	const u8 *payload;
+	u8 hmac[HMAC_SIZE];
+};
+
 struct route_step {
 	enum route_next_case nextcase;
 	struct onionpacket *next;