dde_rump: implement rumpuser_dprintf()

Though we do not actually need this function, it is helpful while
debugging.

Fixes #1141.

repos/dde_rump/src/lib/rump/dummies.cc

@@ -35,7 +35,6 @@ DUMMY(-1, rumpuser_anonmmap)
 DUMMY(-1, rumpuser_close)
 DUMMY(-1, rumpuser_daemonize_begin)
 DUMMY(-1, rumpuser_daemonize_done)
-DUMMY(-1, rumpuser_dprintf)
 DUMMY(-1, rumpuser_iovread)
 DUMMY(-1, rumpuser_iovwrite)
 DUMMY(-1, rumpuser_kill)

repos/dde_rump/src/lib/rump/io.cc

@@ -340,3 +340,13 @@ void rump_io_backend_sync()
 	backend()->submit();
 }
 
+
+void rumpuser_dprintf(const char *fmt, ...)
+{
+	va_list list;
+	va_start(list, fmt);
+
+	Genode::vprintf(fmt, list);
+
+	va_end(list);
+}