checkpatch: don't complain on _Generic() use

Improve CamelCase recognition logic to avoid reporting on
 _Generic() use.

Other C keywords, such as _Bool, are intentionally omitted, as those
should be rather avoided in new source code.

Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>