dsa: Include linux/if_ether.h to fix build error

Include linux/if_ether.h to fix below build errors:

  CC      arch/arm/mach-kirkwood/common.o
In file included from arch/arm/mach-kirkwood/common.c:19:
include/net/dsa.h: In function 'dsa_uses_dsa_tags':
include/net/dsa.h:192: error: 'ETH_P_DSA' undeclared (first use in this function)
include/net/dsa.h:192: error: (Each undeclared identifier is reported only once
include/net/dsa.h:192: error: for each function it appears in.)
include/net/dsa.h: In function 'dsa_uses_trailer_tags':
include/net/dsa.h:197: error: 'ETH_P_TRAILER' undeclared (first use in this function)
make[1]: *** [arch/arm/mach-kirkwood/common.o] Error 1
make: *** [arch/arm/mach-kirkwood] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>