powercap: include header to fix -Wmissing-prototypes

powercap: include header to fix -Wmissing-prototypes

Include the linux/idle_inject.h header to fix W=1 build warning:

    drivers/powercap/idle_inject.c:152:6: warning: no previous prototype for ‘idle_inject_set_duration’ [-Wmissing-prototypes]
    drivers/powercap/idle_inject.c:167:6: warning: no previous prototype for ‘idle_inject_get_duration’ [-Wmissing-prototypes]
    drivers/powercap/idle_inject.c:179:6: warning: no previous prototype for ‘idle_inject_set_latency’ [-Wmissing-prototypes]
    drivers/powercap/idle_inject.c:195:5: warning: no previous prototype for ‘idle_inject_start’ [-Wmissing-prototypes]
    drivers/powercap/idle_inject.c:227:6: warning: no previous prototype for ‘idle_inject_stop’ [-Wmissing-prototypes]
    drivers/powercap/idle_inject.c:299:28: warning: no previous prototype for ‘idle_inject_register’ [-Wmissing-prototypes]
    drivers/powercap/idle_inject.c:345:6: warning: no previous prototype for ‘idle_inject_unregister’ [-Wmissing-prototypes]

Signed-off-by: Pujin Shi <shipj@lemote.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>