Add the precompiled headers to the project info

QMake passes those headers directly to gcc, so the user doesn't need to
include themselves.

src/plugins/cpptools/cppmodelmanagerinterface.h

@@ -76,6 +76,7 @@ public:
         QStringList sourceFiles;
         QStringList includePaths;
         QStringList frameworkPaths;
+        QStringList precompiledHeaders;
     };
 
     class WorkingCopy