usb: gadget: nokia: include f_ecm.c

recent changes to gadget framework have
exposed a flaw in nokia.c where it uses
f_ecm.c but it wasn't including it like
other gadget drivers.

This is a temporary patch until all other
function drivers are converted to new API
where each function driver becomes a module
and binding is done through configfs.

Solves a linking error when building g_nokia.ko

Signed-off-by: Felipe Balbi <balbi@ti.com>