mkhash: fix build errors on FreeBSD 13.0

be64enc, be16dec, and be32dec are declared on FreeBSD 13.0, in
/usr/include/sys/endian.h so we should not declare them.

Fixes the following error during feeds update:
staging_dir/host/bin/mkhash: No such file or directory

gcc scripts/mkhash.c
scripts/mkhash.c:111:1: error: redefinition of 'be64enc'
  111 | be64enc(void *buf, uint64_t u)

Signed-off-by: Georgi Valkov <gvalkov@abv.bg>

scripts/mkhash.c

@@ -105,7 +105,6 @@ be32enc(void *buf, uint32_t u)
 	p[2] = ((uint8_t) ((u >> 8) & 0xff));
 	p[3] = ((uint8_t) (u & 0xff));
 }
-#endif
 
 static void
 be64enc(void *buf, uint64_t u)
@@ -132,6 +131,7 @@ be32dec(const void *buf)
 
 	return (((uint32_t) be16dec(p)) << 16) | be16dec(p + 2);
 }
+#endif
 
 #define MD5_DIGEST_LENGTH	16