Fix compilation when using g++ with libc++

Fixes #1110

include/internal/catch_tostring.h

@@ -26,7 +26,7 @@
 
 
 // We need a dummy global operator<< so we can bring it into Catch namespace later
-struct Catch_global_namespace_dummy;
+struct Catch_global_namespace_dummy {};
 std::ostream& operator<<(std::ostream&, Catch_global_namespace_dummy);
 
 namespace Catch {