Pass buttons state in scroll events

A scroll event might be produced when a mouse button is pressed (for
example when scrolling while selecting a text). For consistency, pass
the actual buttons state (instead of 0).

In practice, it seems that this use case does not work properly with
Android event injection, but it will work with HID mouse.

app/src/control_msg.h

@@ -70,6 +70,7 @@ struct control_msg {
             struct sc_position position;
             int32_t hscroll;
             int32_t vscroll;
+            enum android_motionevent_buttons buttons;
         } inject_scroll_event;
         struct {
             enum android_keyevent_action action; // action for the BACK key