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>

drivers/net/ethernet/nxp/lpc_eth.c

@@ -14,6 +14,7 @@
 #include <linux/crc32.h>
 #include <linux/etherdevice.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/of_net.h>
 #include <linux/phy.h>
 #include <linux/platform_device.h>