CppTools: Change default of "Ignore precompiled headers" to false

If the project has no PCH it has no effect, and if it does have PCH it is
typically important to use it.

Change-Id: I20ce2a0c5713253d2754d31e7b7dca51696b208b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cppcodemodelsettings.h

@@ -77,7 +77,7 @@ signals:
     void changed();
 
 private:
-    PCHUsage m_pchUsage = PchUse_None;
+    PCHUsage m_pchUsage = PchUse_BuildSystem;
     bool m_interpretAmbigiousHeadersAsCHeaders = false;
     bool m_skipIndexingBigFiles = true;
     int m_indexerFileSizeLimitInMB = 5;