Make switch_ioscheduler() return 0 if FIO_HAVE_IOSCHED_SWITCH is undefined

Make switch_ioscheduler() return 0 if FIO_HAVE_IOSCHED_SWITCH is undefined

Defining FIO_HAVE_IOSCHED_SWITCH currently equals saying it's Linux,
as switch_ioscheduler() only works on Linux kernel with "scheduler"
sysfs entry (though read/write to sysfs obviously compiles on others).

This commit makes the function return 0 if FIO_HAVE_IOSCHED_SWITCH
is undefined (i.e. if not Linux). This is essentially the same as
{diskutil,cgroup,blktrace}.c being compiled only on Linux.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>