io_getevents.2: Use syscall(SYS_...); for system calls without a wrapper

io_getevents.2: Use syscall(SYS_...); for system calls without a wrapper

In this case there's a wrapper provided by libaio,
but this page documents the raw syscall.

Also remove <linux/time.h> from the includes: 'struct timespec'
is already documented in system_data_types(7), where the
information is more up to date.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>