usb: gadget: f_fs: resurect usb_functionfs_descs_head structure

Even though usb_functionfs_descs_head structure is now deprecated,
it has been used by some user space tools.  Its removel in commit
[ac8dde11

: “Add flags to descriptors block”] was an oversight
leading to build breakage for such tools.

Bring it back so that old user space tools can still be build
without problems on newer kernel versions.

Cc: <stable@vger.kernel.org>  # 3.14
Reported-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Reported-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>