net: lpc-enet: allow compile testing

The lpc-enet driver can now be built on all platforms, so
allow compile testing as well.

Add one missing header inclusion that is required in some
configurations.

Link: https://lore.kernel.org/r/20190809144043.476786-11-arnd@arndb.de


Signed-off-by: Arnd Bergmann <arnd@arndb.de>