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/combo.h

@@ -647,7 +647,7 @@ protected:
     // used to prevent immediate re-popupping in case closed popup
     // by clicking on the combo control (needed because of inconsistent
     // transient implementation across platforms).
-    wxLongLong              m_timeCanAcceptClick;
+    wxMilliClock_t          m_timeCanAcceptClick;
 
     // how much popup should expand to the left/right of the control
     wxCoord                 m_extLeft;