htl: Make pthread_mutex_t pointer-aligned

This is for future-proofing. On i386, it is 4-byte aligned anyway, but
on x86_64, we want it 8-byte aligned, not 4-byte aligned.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230214173722.428140-4-bugaevc@gmail.com>