fixes a problem with clients which define/request a Strut (see

http://standards.freedesktop.org/wm-spec/wm-spec-1.3.html#id2507548), eg
kicker: after killing/closing kicker the availableWorkspaceArea is not
recalculated which lead to kind of invisible barrier where the Strut was.

src/WinClient.cc

@@ -626,6 +626,7 @@ void WinClient::setStrut(Strut *strut) {
 void WinClient::clearStrut() {
     if (m_strut != 0) {
         screen().clearStrut(m_strut);
+        screen().updateAvailableWorkspaceArea();
         m_strut = 0;
     }
 }