Separate io_u from ioengine [3/3] - rename ioengine.h to ioengines.h

This is based on the previous commits which added a new header
io_u.h and separated io_u related from ioengine.h and ioengines.c.

Since now that ioengine.h and ioengines.c are 1:1 in their contents
(i.e. prototypes and implementation), they should be based on the
same name as well.

This could possibly break outbox ioengines if they include ioengine.h,
but all the inbox ones just include fio.h (probably because that's
what engines/skeleton_external.c does), so the impact is probably
limited, and fio doesn't guarantee anything to outbox ones anyway.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>