[base16] Fix comparison of signed and unsigned integers

gcc 4.7.1 fails to report this erroneous comparison unless assertions
are enabled.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/base16.c

 
 	DBG ( "Base16-decoded \"%s\" to:\n", encoded );
 	DBG_HDA ( 0, raw, len );
-	assert ( len <= base16_decoded_max_len ( encoded ) );
+	assert ( len <= ( int ) base16_decoded_max_len ( encoded ) );
 
 	return len;
 }