[PyQt] Fix to build PyQt5 with Qt5 that doesn't enable session manager

[Philip Balister]

--- PyQt5_gpl-5.8.2/sip/QtGui/qguiapplication.sip.orig  2017-05-13
13:28:42.523392315 -0400
+++ PyQt5_gpl-5.8.2/sip/QtGui/qguiapplication.sip       2017-05-13
13:30:08.081207211 -0400
@@ -270,12 +270,14 @@
 %End

 public:
+%If (PyQt_SessionManager)
 %If (Qt_5_6_0 -)
     static bool isFallbackSessionManagementEnabled();
 %End
 %If (Qt_5_6_0 -)
     static void setFallbackSessionManagementEnabled(bool);
 %End
+%End

 signals:
 %If (Qt_5_6_0 -)