Utils: Fix compile for older Xcode

Amends e38410b76c.

Change-Id: I57dd8a3dfa584ec2f2f8836700bfd31af95c3a4f
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/persistentsettings.cpp

@@ -108,7 +108,7 @@ namespace Utils {
 
 struct Context // Basic context containing element name string constants.
 {
-    Context() = default;
+    Context() {}
     const QString qtCreatorElement = QString("qtcreator");
     const QString dataElement = QString("data");
     const QString variableElement = QString("variable");