sandbox: mark local functions static

seunshare.c:92:6: warning: no previous prototype for ‘handler’ [-Wmissing-prototypes]
       92 | void handler(int sig) {
          |      ^~~~~~~

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

sandbox/seunshare.c

@@ -89,7 +89,7 @@ static int drop_privs(uid_t uid)
 /**
  * If the user sends a siginto to seunshare, kill the child's session
  */
-void handler(int sig) {
+static void handler(int sig) {
 	if (child > 0) kill(-child,sig);
 }