blk/aio: fix mismatched parenthesis in POSIX AIO submit path

The condition `if ((cur->n_aiocb == 1) {` has an unbalanced
parenthesis and fails to compile. This code is only built on the
HAVE_POSIXAIO (BSD) path, so the breakage is normally hidden.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>