QTestLog: use the right preprocessor test for C++ features

Use ifdef, not if, otherwise we trigger -Wundef if the feature
is unsupported.

Pick-to: 6.9 6.8
Task-number: QTBUG-132900
Change-Id: I8a779da59bf2c7add14ace6405faf19b7ca48c28
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/testlib/qtestlog.cpp

@@ -113,7 +113,7 @@ public:
     }
 
 private:
-#if __cpp_lib_atomic_shared_ptr
+#ifdef __cpp_lib_atomic_shared_ptr
     SharedLoggersContainer load() const { return loggers.load(std::memory_order_relaxed); }
     void store(SharedLoggersContainer newLoggers)
     {