selftests/bpf: Add static to enable_all_controllers()

Add static to enable_all_controllers() to get rid from annoying warning
during samples/bpf build:

samples/bpf/../../tools/testing/selftests/bpf/cgroup_helpers.c:44:5:
warning: no previous prototype for ‘enable_all_controllers’
[-Wmissing-prototypes]
 int enable_all_controllers(char *cgroup_path)

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191002120404.26962-2-ivan.khoronzhuk@linaro.org