usb: fix usb_fdt_support.c when altq enabled (usb_ehernet.h changes)

After r351243 when ALTQ was enabled in the kernel, the inline functions
in ifq.h would not have full type information as if_var.h was not
included.

Given usb_ethernet.h already includes all the various headers (which)
is the cause of the problem here, add if_var.h to it. This fixes the
builds again.

Reported by: CI system, e.g. FreeBSD-head-aarch64-LINT