Revert "add __dummy field to HDNode where fingerprint used to be (in order not to binary compatibility)"

This reverts commit 9a8df5a4bb.

bip32.h

@@ -36,7 +36,6 @@ typedef struct {
 
 typedef struct {
 	uint32_t depth;
-	uint32_t __dummy;
 	uint32_t child_num;
 	uint8_t chain_code[32];
 	uint8_t private_key[32];