V4L/DVB (4342): Fix ext_controls align on 64 bit architectures

u64 is aligned as 128bits on x86_64 architetures, requiring an special
handling to ioctls that depends on v4l2_ext_control. 
Let's fix this before ext controls go to kernel mainstream to avoid one 
more compat32 stuff.
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>

include/linux/videodev2.h

@@ -716,7 +716,7 @@ struct v4l2_ext_control
 		__s64 value64;
 		void *reserved;
 	};
-};
+} __attribute__ ((packed));
 
 struct v4l2_ext_controls
 {