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

src/libcharon/encoding/payloads/encryption_payload.h

@@ -29,11 +29,6 @@ typedef struct encryption_payload_t encryption_payload_t;
 #include <crypto/aead.h>
 #include <encoding/payloads/payload.h>
 
-/**
- * Encrpytion payload length in bytes without IV and following data.
- */
-#define ENCRYPTION_PAYLOAD_HEADER_LENGTH 4
-
 /**
  * The encryption payload as described in RFC section 3.14.
  */