ethtool: Fix build due to lack of ethtool.h include.

net/core/ethtool.c: In function 'ethtool_get_regs':
net/core/ethtool.c:818:2: error: implicit declaration of function 'vmalloc'
net/core/ethtool.c:818:9: warning: assignment makes pointer from integer without a cast
net/core/ethtool.c:833:2: error: implicit declaration of function 'vfree'

Signed-off-by: David S. Miller <davem@davemloft.net>