We don't call this function if CONFIG_DEBUG_FS is not defined, but we
should not be compiling it either, as the declaration of the debugfs
core functions is not included.

Reported by the kbuild test robot.
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>