CppTools: Fix build with MSVC

compileroptionsbuilder.cpp(196): error C2440: 'initializing': cannot
  convert from 'const char *' to 'QString'

Change-Id: Ifa5e50ab927c534a855c2f7871d3dce87092b30a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cpptools/compileroptionsbuilder.cpp

@@ -192,8 +192,8 @@ static QStringList createLanguageOptionGcc(ProjectFile::Kind fileKind, bool objc
 void CompilerOptionsBuilder::addWordWidth()
 {
     const QString argument = m_projectPart.toolChainWordWidth == ProjectPart::WordWidth64Bit
-                                 ? "-m64"
-                                 : "-m32";
+                                 ? QLatin1String("-m64")
+                                 : QLatin1String("-m32");
     add(argument);
 }