netdev-linux: Fix broken build on RHEL 6.

netdev-linux: Fix broken build on RHEL 6.

Commit 00fa9d37c2b "Do not include net/ethernet.h and linux/if_tunnel.h"
introduced a compile error on RHEL 6:

lib/netdev-linux.c: In function 'netdev_linux_listen':
lib/netdev-linux.c:734: error: 'ETH_P_ALL' undeclared (first use in this
function)

This fixes the problem.

I verified that the Android NDK r6b mentioned in the previous commit
contains a file named android-ndk-r6b/platforms/android-3/arch-x86/use/
linux/if_ether.h that defines ETH_P_ALL.  I didn't try building on that
platform.