headers, pppox: Add missing #include to <linux/if_pppox.h>

<linux/if_ppox.h> uses ETH_ALEN, defined in <linux/if_ether.h>.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_pppox.h

@@ -20,8 +20,8 @@
 #include <linux/types.h>
 #include <asm/byteorder.h>
 
-#ifdef  __KERNEL__
 #include <linux/if_ether.h>
+#ifdef  __KERNEL__
 #include <linux/if.h>
 #include <linux/netdevice.h>
 #include <linux/ppp_channel.h>