ProjectExplorer: Fix missing include

Change-Id: I7a113418205485b1f616d4b92666a8fc92edd399
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/projectexplorer/currentprojectfilter.cpp

@@ -31,6 +31,7 @@
 #include "currentprojectfilter.h"
 #include "projecttree.h"
 #include "project.h"
+#include "session.h"
 
 #include <utils/algorithm.h>