compat.h: Add if_ether.h symbols for older kernels.

This patch fixes these build errors on CentOS 6.

  CC	net/ether.o
net/ether.c:12: error: ‘ETH_P_BATMAN’ undeclared here (not in a function)
net/ether.c:19: error: ‘ETH_P_LINK_CTL’ undeclared here (not in a function)
net/ether.c:21: error: ‘ETH_P_8021AD’ undeclared here (not in a function)
net/ether.c:21: error: ‘ETH_P_802_EX1’ undeclared here (not in a function)
net/ether.c:22: error: ‘ETH_P_8021AH’ undeclared here (not in a function)
net/ether.c:22: error: ‘ETH_P_MVRP’ undeclared here (not in a function)
net/ether.c:23: error: ‘ETH_P_QINQ1’ undeclared here (not in a function)
net/ether.c:24: error: ‘ETH_P_QINQ2’ undeclared here (not in a function)
net/ether.c:24: error: ‘ETH_P_QINQ3’ undeclared here (not in a function)
net/ether.c:31: error: ‘ETH_P_CANFD’ undeclared here (not in a function)
net/ether.c:34: error: ‘ETH_P_CAIF’ undeclared here (not in a function)

Signed-off-by: Vinson Lee <vlee@twitter.com>