usb: gadget: uvc: Minimize #include in headers

In order to speed up compilation, only include the headers that are
strictly required within other headers. To that end, use forward
structure declaration and move #include statements to .c file as
appropriate.

While at it, sort headers alphabetically, and remove unneeded __KERNEL__
guards.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>