usb: gadget: add missing #include's

When #include'd alone, <linux/usb/gadget.h>
causes a lot of compilation errors and warnings
-- all because it relies on the including code to
bring in the necessary #include's instead of
doing this itself.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>