rijndael: fix compiler warning on aarch64

* cipher/rijndael.c (do_setkey): Use braces for empty if statement
instead of semicolon.

Patch fixes following warning:

rijndael.c: In function 'do_setkey':
rijndael.c:507:9: warning: suggest braces around empty body in an 'if' statement [-Wempty-body]

;
^

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>