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>