[dnssd] add stub/empty implementation for new otPlatDnssd APIs (#2818)

Adds empty implementation for `otPlatDnssdStartRecordQuerier()` and
`otPlatDnssdStopRecordQuerier()` in the `posix/dnssd.cpp` file.

Author: abtink

src/host/posix/dnssd.cpp

@@ -181,6 +181,18 @@ void otPlatDnssdStopIp4AddressResolver(otInstance *aInstance, const otPlatDnssdA
     OTBR_UNUSED_VARIABLE(aResolver);
 }
 
+void otPlatDnssdStartRecordQuerier(otInstance *aInstance, const otPlatDnssdRecordQuerier *aQuerier)
+{
+    OTBR_UNUSED_VARIABLE(aInstance);
+    OTBR_UNUSED_VARIABLE(aQuerier);
+}
+
+void otPlatDnssdStopRecordQuerier(otInstance *aInstance, const otPlatDnssdRecordQuerier *aQuerier)
+{
+    OTBR_UNUSED_VARIABLE(aInstance);
+    OTBR_UNUSED_VARIABLE(aQuerier);
+}
+
 //----------------------------------------------------------------------------------------------------------------------
 
 namespace otbr {