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

src/libcharon/encoding/payloads/certreq_payload.h

@@ -28,11 +28,6 @@ typedef struct certreq_payload_t certreq_payload_t;
 #include <encoding/payloads/payload.h>
 #include <encoding/payloads/cert_payload.h>
 
-/**
- * Length of a CERTREQ payload without the CERTREQ data in bytes.
- */
-#define CERTREQ_PAYLOAD_HEADER_LENGTH 5
-
 /**
  * Class representing an IKEv2 CERTREQ payload.
  *