Update preconditions for execsnoop.
Update preconditions for execsnoop.

bpf_get_current_task is only available in 4.8 and above.

Signed-off-by: David Calavera <b9b0c8e7f90ecf337f6797c4d2e0f1dd0dbc0324@gmail.com>

tuxology/bcc,iovisor/bcc,mcaleavya/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc,tuxology/bcc,iovisor/bcc,brendangregg/bcc,mcaleavya/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc,mcaleavya/bcc,mcaleavya/bcc,iovisor/bcc,mcaleavya/bcc