Fix warnings on non-static data member initializers (C++11 feature)

We're not formally using C++11 yet so those trigger compilation warnings
(at least with GCC 5):

./main/input_default.h:122:30: warning: non-static data member initializers only available with -std=c++11 or -std=gnu++11
  CursorShape default_shape = CURSOR_ARROW;
                              ^

Note: We may allow those eventually (especially for non-int static const),
but most of current occurrences were inconsistent with all other classes.

See also http://www.stroustrup.com/C++11FAQ.html#member-init

main/input_default.cpp

@@ -636,6 +636,7 @@ InputDefault::InputDefault() {
 	emulate_mouse_from_touch = false;
 	mouse_from_touch_index = -1;
 	main_loop = NULL;
+	default_shape = CURSOR_ARROW;
 
 	hat_map_default[HAT_UP].type = TYPE_BUTTON;
 	hat_map_default[HAT_UP].index = JOY_DPAD_UP;