Add eBPF syscall requirement to android-4.9

eBPF syscall is required for android-4.9 kernel since the new network
monitoring tool and cgroup bpf tool will be depend on it.

Bug: 30950746
Bug: 36510518
Test: bpf_test.py in kernel/test/net/test/ pass
Change-Id: I8ad11e1ee80c568d38f3d022959b28040c07bf03
Signed-off-by: Chenbo Feng <fengc@google.com>