Use wxMilliClock_t to store values returned by wxGetLocalTimeMillis().

This fixes wxComboCtrl compilation with wxUSE_LONGLONG==0 as wxMilliClock_t is
always mapped to the proper base type and hence can be used even if wxLongLong
type is not defined.

See #16939.

include/wx/msw/combo.h

@@ -100,7 +100,7 @@ private:
 
 #if wxUSE_COMBOCTRL_POPUP_ANIMATION
     // Popup animation related
-    wxLongLong  m_animStart;
+    wxMilliClock_t m_animStart;
     wxTimer     m_animTimer;
     wxRect      m_animRect;
     int         m_animFlags;