ExternalTool: Add missing member to assignment operator

Detected by CppCheck

Change-Id: I6a5a7b5707619e3588d8a52d3fd31ffc446a5ca1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/externaltool.cpp

@@ -128,6 +128,7 @@ ExternalTool &ExternalTool::operator=(const ExternalTool &other)
     m_errorHandling = other.m_errorHandling;
     m_modifiesCurrentDocument = other.m_modifiesCurrentDocument;
     m_fileName = other.m_fileName;
+    m_presetFileName = other.m_presetFileName;
     m_presetTool = other.m_presetTool;
     return *this;
 }