KVM: uses TASKSTATS, depends on NET

CONFIG_TASKSTATS just had a change to use netlink, including
a change to "depends on NET".  Since "select" does not follow
dependencies, KVM also needs to depend on NET to prevent build
errors when CONFIG_NET is not enabled.

Sample of the reported "undefined reference" build errors:

taskstats.c:(.text+0x8f686): undefined reference to `nla_put'
taskstats.c:(.text+0x8f721): undefined reference to `nla_reserve'
taskstats.c:(.text+0x8f8fb): undefined reference to `init_net'
taskstats.c:(.text+0x8f905): undefined reference to `netlink_unicast'
taskstats.c:(.text+0x8f934): undefined reference to `kfree_skb'
taskstats.c:(.text+0x8f9e9): undefined reference to `skb_clone'
taskstats.c:(.text+0x90060): undefined reference to `__alloc_skb'
taskstats.c:(.text+0x901e9): undefined reference to `skb_put'
taskstats.c:(.init.text+0x4665): undefined reference to `genl_register_family'
taskstats.c:(.init.text+0x4699): undefined reference to `genl_register_ops'
taskstats.c:(.init.text+0x4710): undefined reference to `genl_unregister_ops'
taskstats.c:(.init.text+0x471c): undefined reference to `genl_unregister_family'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>