base: add remove_client to rm_session

Fixes #13

base-linux/include/base/platform_env.h

@@ -252,6 +252,8 @@ namespace Genode {
 					Pager_capability add_client(Thread_capability thread) {
 						return Pager_capability(); }
 
+					void remove_client(Pager_capability pager) { }
+
 					void fault_handler(Signal_context_capability handler) { }
 
 					State state() { return State(); }