Fix missing prototypes

i3 will now compile with no warnings when -Wmissing-prototypes is used.

testcases/inject_randr1.5.c

@@ -34,7 +34,7 @@ static void read_server_x11_packet_cb(EV_P_ ev_io *w, int revents);
 
 static char *sun_path = NULL;
 
-void cleanup_socket(void) {
+static void cleanup_socket(void) {
     if (sun_path != NULL) {
         unlink(sun_path);
         free(sun_path);