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.

test/testrendertarget.c

@@ -173,7 +173,7 @@ Draw(DrawState *s)
     return SDL_TRUE;
 }
 
-void loop()
+void loop(void)
 {
     int i;
     SDL_Event event;