Warning was:
In file included from bridge.c:16:0:
../include/namespace.h:33:12: warning: ‘setns’ defined but not used [-Wunused-function]

CC: Vadim Kochan <vadim4j@gmail.com>
Fixes: eb67e449 ("lib: Add netns_switch func for change network namespace")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>