compat.h: fix a spelling error in <linux/compat.h>

There is no compat_sys_readv64v2 syscall, only a compat_sys_preadv64v2
one.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>