Unify the include guards accross all header files

src/nft.h

@@ -1,3 +1,6 @@
+#ifndef _NFT_H_
+#define _NFT_H_
+
 #include "tokens.h"
 
 #define COLLECTION_NAME_MAX_LEN 70
@@ -6,3 +9,5 @@ typedef struct nftInfo_t {
     uint8_t contractAddress[ADDRESS_LENGTH];  // must be first item
     char collectionName[COLLECTION_NAME_MAX_LEN + 1];
 } nftInfo_t;
+
+#endif  // _NFT_H_