bpfilter: fix build dependency

BPFILTER could have been enabled without INET causing this build error:
ERROR: "bpfilter_process_sockopt" [net/bpfilter/bpfilter.ko] undefined!

Fixes: d2ba09c1 ("net: add skeleton of bpfilter kernel module")
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bpfilter/Kconfig

 menuconfig BPFILTER
 	bool "BPF based packet filtering framework (BPFILTER)"
 	default n
-	depends on NET && BPF
+	depends on NET && BPF && INET
 	help
 	  This builds experimental bpfilter framework that is aiming to
 	  provide netfilter compatible functionality via BPF