PPCallback clang plugin should say so

Change-Id: I76148f152f57af74935c99e290b30b509511dfd8
Reviewed-on: https://gerrit.libreoffice.org/72546
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

compilerplugins/clang/readability-redundant-pp.cxx

@@ -40,6 +40,10 @@ public:
     void Ifdef(clang::SourceLocation aLoc, const clang::Token& rMacroNameTok,
                const clang::MacroDefinition& rMacroDefinition) override;
     void Endif(clang::SourceLocation aLoc, clang::SourceLocation aIfLoc) override;
+    enum
+    {
+        isPPCallback = true
+    };
 
 private:
     clang::Preprocessor& m_rPP;