Don't use wxVector<T>::clear() when T is an incomplete type

At least the OpenVMS compiler doesn't like it and we can easily avoid
doing this by simply moving ClearExtraAccels() definition to the source
file.

include/wx/menuitem.h

@@ -131,7 +131,7 @@ public:
     // return vector of extra accels. Implementation only.
     const wxVector<wxAcceleratorEntry>& GetExtraAccels() const { return m_extraAccels; }
 
-    virtual void ClearExtraAccels() { m_extraAccels.clear(); }
+    virtual void ClearExtraAccels();
 #endif // wxUSE_ACCEL
 
 #if WXWIN_COMPATIBILITY_2_8

src/common/menucmn.cpp

@@ -301,6 +301,11 @@ void wxMenuItemBase::AddExtraAccel(const wxAcceleratorEntry& accel)
     m_extraAccels.push_back(accel);
 }
 
+void wxMenuItemBase::ClearExtraAccels()
+{
+    m_extraAccels.clear();
+}
+
 #endif // wxUSE_ACCEL
 
 void wxMenuItemBase::SetItemLabel(const wxString& str)