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/checkkeysthreads.c

@@ -103,7 +103,7 @@ print_modifiers(char **text, size_t *maxlen)
 }
 
 static void
-PrintModifierState()
+PrintModifierState(void)
 {
     char message[512];
     char *spot;
@@ -162,7 +162,7 @@ PrintText(const char *eventtype, const char *text)
     SDL_Log("%s Text (%s): \"%s%s\"\n", eventtype, expanded, *text == '"' ? "\\" : "", text);
 }
 
-void loop()
+void loop(void)
 {
     SDL_Event event;
     /* Check for events */