problem building dev/e1000

[Eric Joyner]

ifdi_if.h is a system generated interface file that network drivers that
depend on iflib use. It's generated from ifdi_if.m in sys/net.

My guess is that you don't have "device iflib" defined in your kernel
configuration. em(4) depends on iflib in FreeBSD 12/13, and with r343617,
iflib is now explicitly a module that needs to be included in your kernel
config if you want to use a driver that depends on it. It's already in the
GENERIC kernel for amd64; are you using a kernel config file that doesn't
inherit from GENERIC?

- Eric