Fix warning on 64bit

Because the template instantiation rules of C++ do not deal well with
null pointers specified as '0', the constructor of 'Local_addr' was
instantiated for [T = int], which does not make sense. To avoid the
warning "cast to pointer from integer of different size", we need to
explicitly state that '0' is a pointer. In C++11, there is the 'nullptr'
keyword, but until we switch to this version, we have to state (void *)0.

base/include/rm_session/rm_session.h

@@ -129,7 +129,7 @@ namespace Genode {
 		virtual Local_addr attach(Dataspace_capability ds,
 		                          size_t size = 0, off_t offset = 0,
 		                          bool use_local_addr = false,
-		                          Local_addr local_addr = 0,
+		                          Local_addr local_addr = (void *)0,
 		                          bool executable = false) = 0;
 
 		/**