powercap: idle_inject: Export symbols

Export symbols for external interfaces, so that they can be used in
other loadable modules.

Export is done under name space IDLE_INJECT.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>