Add empty function definitions

Add definitions for every function that does nothing for now.
-> now able to implement and call those functions already

src/input.c

@@ -1 +1,5 @@
-#include "input.h"
+#include "input.h"
+
+void processInputEvent(){
+    return;
+}

src/menu.c

@@ -1 +1,18 @@
-#include "menu.h"
+#include "menu.h"
+
+
+void showStartScreen(){
+    return;
+}
+
+void showLeaderboard(){
+    return;
+}
+
+void showSettings(){
+    return;
+}
+
+void menuHandleInput(int event){
+    return;
+}

src/render.c

@@ -1 +1,5 @@
 #include "render.h"
+
+void renderGame(){
+    return;
+}

src/snake.c

@@ -1 +1,25 @@
-#include "snake.h"
+#include "snake.h"
+
+void snakeInit(){
+    return;
+}
+
+void snakeGrow(){
+    return;
+}
+
+void snakeMove(){
+    return;
+}
+
+void snakeSetDir(snakeDirection_t dir){
+    return;
+}
+
+bool snakeIsAlive(){
+    return true;
+}
+
+void snakeSetHeadPos(){
+    return;
+}