Add missing --cmdhelp type string for FIO_OPT_UNSUPPORTED

Without "unsupported" at index 12 (FIO_OPT_UNSUPPORTED) of typehelp[],
unsupported commands print irrelevant binary at typehelp[12].

 # fio --cmdhelp=numa_cpu_nodes
       numa_cpu_nodes: Build fio with libnuma-dev(el) to enable this option
                 type: ÏÿF
              default: no default

With this commit.
 # fio --cmdhelp=numa_cpu_nodes
       numa_cpu_nodes: Build fio with libnuma-dev(el) to enable this option
                 type: unsupported
              default: no default

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