Remove global qualifier from struct definition

Global qualifier in struct/class name definition is accepted by Clang but not allowed in GCC (which apparently doesn't implement the correction of the C++ Core Issue 355, see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3259.htm).
Removing the global qualifier solved the issue.

Re ECFLOW-1922

ACore/test/TestConverter.cpp

@@ -27,7 +27,7 @@ struct Gizmo
 };
 
 template <>
-struct ::ecf::converter_traits<Widget, Gizmo>
+struct ecf::converter_traits<Widget, Gizmo>
 {
     inline static Gizmo convert(const Widget&) { return Gizmo{}; }
 };