include/uapi/linux/if_pppox.h: include linux/if.h

Fixes userspace compilation error:

error: ‘IFNAMSIZ’ undeclared here (not in a function)
Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_pppox.h

@@ -21,6 +21,7 @@
 #include <asm/byteorder.h>
 
 #include <linux/socket.h>
+#include <linux/if.h>
 #include <linux/if_ether.h>
 #include <linux/if_pppol2tp.h>