tools: usb: ffs-test: Fix build failure

Fixes this build failure:
gcc -Wall -Wextra -g -lpthread -I../include -o testusb testusb.c
gcc -Wall -Wextra -g -lpthread -I../include -o ffs-test ffs-test.c
In file included from ffs-test.c:41:0:
../../include/linux/usb/functionfs.h:4:39: fatal error:
uapi/linux/usb/functionfs.h: No such file or directory
compilation terminated.
make: *** [ffs-test] Error 1

Signed-off-by: Maxin B. John <maxin.john@enea.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: stable <stable@vger.kernel.org>  # 3.7+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>