netdevsim: make functions nsim_bpf_create_prog and nsim_bpf_destroy_prog static

Functions nsim_bpf_create_prog and nsim_bpf_destroy_prog are local to the
source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'nsim_bpf_create_prog' was not declared. Should it be static?
symbol 'nsim_bpf_destroy_prog' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>