[ipv6] Fix compilation under FreeBSD

Reported-by: Dewey Hylton <dewey@hyltown.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/ndp.h

 } __attribute__ (( packed ));
 
 /** NDP option block size */
-#define NDP_OPTION_BLKSZ 8
+#define NDP_OPTION_BLKSZ 8U
 
 /** NDP source link-layer address option */
 #define NDP_OPT_LL_SOURCE 1