net: fix test_bpf build to depend on NET

Fix build when CONFIG_NET is not enabled.
Fixes these build errors:

WARNING: "sk_unattached_filter_destroy" [lib/test_bpf.ko] undefined!
WARNING: "kfree_skb" [lib/test_bpf.ko] undefined!
WARNING: "sk_unattached_filter_create" [lib/test_bpf.ko] undefined!
WARNING: "sk_run_filter_int_skb" [lib/test_bpf.ko] undefined!
WARNING: "__alloc_skb" [lib/test_bpf.ko] undefined!

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>