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/testmultiaudio.c

@@ -53,7 +53,7 @@ play_through_once(void *arg, Uint8 *stream, int len)
     }
 }
 
-void loop()
+void loop(void)
 {
     if (SDL_AtomicGet(&cbd[0].done)) {
 #ifdef __EMSCRIPTEN__