Use void arguments list consistently in C code

On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

PUBLISHED_FROM=5519526cf84e2bbd425a726fcc112fea1a95cbf1

examples/ESP8266_RTOS/user/user_main.c

@@ -54,7 +54,7 @@ void ev_handler(struct mg_connection *nc, int ev, void *p) {
   }
 }
 
-void setup_ap() {
+void setup_ap(void) {
   int off = 0;
   struct ip_info info;
   struct softap_config cfg;