Bugfix: Fix htobe64 on big endian platforms that don't define it

It needs to be bswap_64, not bswap_32

miner.h

@@ -145,7 +145,7 @@ static inline int fsync (int fd)
 # if __BYTE_ORDER == __LITTLE_ENDIAN
 #  define be32toh(x) bswap_32(x)
 #  define htobe32(x) bswap_32(x)
-#  define htobe64(x) bswap_32(x)
+#  define htobe64(x) bswap_64(x)
 # elif __BYTE_ORDER == __BIG_ENDIAN
 #  define be32toh(x) (x)
 #  define htobe32(x) (x)