base: fix warning in RPC framework

The recent change of the TRACE session interface triggered the
following warning:

/home/no/src/genode/repos/base/include/base/ipc.h:79:4: warning: ‘ret’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    *reinterpret_cast<T *>(&_sndbuf[_write_offset]) = value;
    ^
In file included from /home/no/src/genode/repos/base/src/core/include/trace/session_component.h:19:0,
                 from /home/no/src/genode/repos/base/src/core/trace_session_component.cc:15:
/home/no/src/genode/repos/base/include/base/rpc_server.h:132:42: note: ‘ret’ was declared here
     typename This_rpc_function::Ret_type ret;

The warning occurs for basic return types (like size_t), which are
indeed not initialized. The variable gets its value assigned by the
corresponding 'call_member' overload, to which the variable is passed as
reference. But the compiler apparently is not able to detect this assignment.

Declaring 'ret' with a C++11-style default initializer fixes the warning.

repos/base/include/base/rpc_server.h

@@ -129,7 +129,7 @@ class Genode::Rpc_dispatcher : public RPC_INTERFACE
 				 */
 				typedef typename This_rpc_function::Exceptions Exceptions;
 
-				typename This_rpc_function::Ret_type ret;
+				typename This_rpc_function::Ret_type ret { };
 				Rpc_exception_code exc;
 				exc = _do_serve(args, ret, Overload_selector<This_rpc_function, Exceptions>());
 				os << ret;