Fix Linux & macOS build in release

msvctoolchain.h/.cpp is only available on Windows (except if tests are
enabled...)

Change-Id: Id2ad5bd0945f1f1c21ca6a3a08e8bc4d5f50c3ed
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/projectexplorer/toolchainmanager.cpp

@@ -251,7 +251,9 @@ bool ToolChainManager::isLanguageSupported(const Core::Id &id)
 
 void ToolChainManager::aboutToShutdown()
 {
+#ifdef Q_OS_WIN
     MsvcToolChain::cancelMsvcToolChainDetection();
+#endif
 }
 
 } // namespace ProjectExplorer