Add a payload.get_header_length() method, remove header length definitions

src/libcharon/encoding/payloads/nonce_payload.h

@@ -32,11 +32,6 @@ typedef struct nonce_payload_t nonce_payload_t;
  */
 #define NONCE_SIZE 32
 
-/**
- * Length of a nonce payload without a nonce in bytes.
- */
-#define NONCE_PAYLOAD_HEADER_LENGTH 4
-
 /**
  * Object representing an IKEv1/IKEv2 Nonce payload.
  */