ClangCodeModel: Fix compile

Change-Id: I00229f3af2ba85ea14677fe47939a75c309a4116
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/clangcodemodel/clangprojectsettingspropertiespage.h

@@ -34,6 +34,10 @@
 
 #include <QString>
 
+namespace ProjectExplorer {
+    class Project;
+}
+
 namespace ClangCodeModel {
 namespace Internal {