Avoid superfluous compiler warnings

GCC warns about uninitialized local variables in cases where no
initialization is needed, in particular in the overloads of the
'Capability::call()' function. Prior this patch, we dealt with those
warnings by using an (unreliable) GCC pragma or by disabling the
particular warning altogether (which is a bad idea). This patch removes
the superfluous warnings by telling the compiler that the variable in
question is volatile.

base-linux/src/base/ipc/ipc.cc

@@ -151,7 +151,7 @@ void Ipc_server::_prepare_next_reply_wait()
 	_read_offset = sizeof(long);
 
 	/* read client thread id from request buffer */
-	long tid;
+	long tid = 0;
 	if (_reply_needed) {
 		_read_from_buf(tid);
 		Ipc_ostream::_dst = Native_capability(tid, 0); /* only _tid member is used */