Add includes to the codemodel specified via PKGCONFIG in .pro files

We run pkg-config --cflags-only-I $$PKGCONFIG in the background to find
those includes.

Task-Nr: 250195

src/plugins/qt4projectmanager/qt4nodes.h

@@ -99,7 +99,8 @@ enum Qt4Variable {
     IncludePathVar,
     CxxCompilerVar,
     UiDirVar,
-    MocDirVar
+    MocDirVar,
+    PkgConfigVar
 };
 
 class Qt4PriFileNode;