Add RFFT2d to optimized only kernel list

This fixes framework test generation when the "optimized"
kernel mode option is not selected. As a result, the
optimized RFFT2d kernel is used when generating tests in
the reference kernel mode to prevent "op not found"
errors.

Signed-off-by: Luke Hutton <luke.hutton@arm.com>
Change-Id: Ice18bfa08c46e237e39c99f849622bf99d9b849c