samples/bpf: add static to function with no prototype

Fixes the following warning

samples/bpf/cookie_uid_helper_example.c: At top level:
samples/bpf/cookie_uid_helper_example.c:276:6: warning: no previous prototype for ‘finish’ [-Wmissing-prototypes]
 void finish(int ret)
      ^~~~~~
  HOSTLD  samples/bpf/per_socket_stats_example

Signed-off-by: Alexander Alemayhu <alexander@alemayhu.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>