libc: qualify Plugin_context as base class

...by defining a virtual destructor.

repos/libports/include/libc-plugin/fd_alloc.h

@@ -33,7 +33,7 @@ namespace Libc {
 	/**
 	 * Plugin-specific file-descriptor context
 	 */
-	class Plugin_context { };
+	struct Plugin_context { virtual ~Plugin_context() { } };
 
 	enum { ANY_FD = -1 };