CppTools: Consistently use nullptr

Fixed by clang-tidy modernize-use-nullptr.

Change-Id: I951627ba3543faa357f67ee76527518290efe3f1
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/cppindexingsupport.h

@@ -67,7 +67,7 @@ public:
 
 
 public:
-    SymbolSearcher(QObject *parent = 0);
+    SymbolSearcher(QObject *parent = nullptr);
     ~SymbolSearcher() override = 0;
     virtual void runSearch(QFutureInterface<Core::SearchResultItem> &future) = 0;
 };