usb: gadget: FunctionFS: share VLA macros with all usb gadget files

Variable Length Array macros allow portable (compilable with both gcc
and clang) way of allocating a number of structures using a single
memory chunk. They can be useful for files other than f_fs.c,
so move them to a header file.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>