lib: fix warning in namespace.h

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>

include/namespace.h

@@ -30,7 +30,7 @@
 #endif
 
 #ifndef HAVE_SETNS
-static int setns(int fd, int nstype)
+static inline int setns(int fd, int nstype)
 {
 #ifdef __NR_setns
 	return syscall(__NR_setns, fd, nstype);