usb: gadget: uvc: Avoid -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the structure. Notice
that `struct uvc_input_header_descriptor` is a flexible structure --a
structure that contains a flexible-array member.

With this, fix three of the following warnings:

drivers/usb/gadget/function/uvc_configfs.h:77:57: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/Z9dyY7_ydJiGqh_d@kspp


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>