Don't malloc ioengine_ops for cpp_null

fio assumes opses aren't dynamically allocated which means malloc'd
engine here never gets freed.

According to 46a67478 and a8075704, the point of get_ioengine() is
how non-static ops gets initialized by g++, but not how space for ops
gets allocated, thus it should just define a static global variable
within "C" scope and pass the address of ops to a caller via **.

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