net: bpf: fix bpf syscall dependence on anon_inodes

minimal configurations where EPOLL, PERF_EVENTS, etc are disabled,
but NET is enabled, are failing to build with link error:
kernel/built-in.o: In function `bpf_prog_load':
syscall.c:(.text+0x3b728): undefined reference to `anon_inode_getfd'

fix it by selecting ANON_INODES when NET is enabled

Reported-by: Michal Sojka <sojkam1@fel.cvut.cz>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>