Fix warning for Android NDK compiler: "function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]"

https://stackoverflow.com/questions/42125/warning-error-function-declaration-isnt-a-prototype
In C int foo() and int foo(void) are different functions. int foo() accepts an arbitrary number of arguments, while int foo(void) accepts 0 arguments. In C++ they mean the same thing.

(cherry picked from commit ccade50587)

test/testgles2.c

@@ -569,7 +569,7 @@ render_thread_fn(void *render_ctx)
 }
 
 static void
-loop_threaded()
+loop_threaded(void)
 {
     SDL_Event event;
     int i;
@@ -598,7 +598,7 @@ loop_threaded()
 #endif
 
 static void
-loop()
+loop(void)
 {
     SDL_Event event;
     int i;