drm/i915/uc: Drop superfluous externs in intel_uc.h

Externs are implicit and we generally try to avoid them.

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>