[2946] get access to base class's static method via 'this' pointer

as some compilers don't allow direct use of Base::method.

src/lib/asiodns/tests/dns_server_unittest.cc

@@ -779,8 +779,7 @@ TEST_F(SyncServerTest, resetUDPServerBeforeEvent) {
     (*udp_server_)();
     udp_server_->stop();
     udp_server_.reset();
-    void (*prev_handler)(int) =
-                std::signal(SIGALRM, DNSServerTestBase::stopIOService);
+    void (*prev_handler)(int) = std::signal(SIGALRM, this->stopIOService);
     current_service = &service;
     alarm(IO_SERVICE_TIME_OUT);
     service.run();