Change .category of cpuio to FIO_OPT_C_ENGINE

Change .category of cpuio to FIO_OPT_C_ENGINE

This is supposed to be ENGINE instead of GENERAL.

 # grep "\.category" engines -rI | grep -v ENGINE
 engines/cpu.c:          .category = FIO_OPT_C_GENERAL,
 engines/cpu.c:          .category = FIO_OPT_C_GENERAL,
 engines/cpu.c:          .category = FIO_OPT_C_GENERAL,

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