libstdc++: Make net::system_context tag type constructor explicit

libstdc++-v3/ChangeLog:

	* include/experimental/executor (system_context::a__tag): Make
	default constructor explicit.

libstdc++-v3/include/experimental/executor

@@ -884,7 +884,7 @@ inline namespace v1
   private:
     friend system_executor;
 
-    struct __tag { };
+    struct __tag { explicit __tag() = default; };
     system_context(__tag) { }
 
     thread			_M_thread;