[mle] fix warning with storedCount (#10301)

This commit fixes the warning, when logs are disabled:

warning: variable "storedCount" set but not used
[-Wunused-but-set-variable]

Signed-off-by: Łukasz Duda <[email protected]>

src/core/thread/mle_router.cpp

@@ -1782,6 +1782,8 @@ Error MleRouter::ProcessAddressRegistrationTlv(RxInfo &aRxInfo, Child &aChild)
     MlrManager::MlrAddressArray oldMlrRegisteredAddresses;
 #endif
 
+    OT_UNUSED_VARIABLE(storedCount);
+
     SuccessOrExit(error =
                       Tlv::FindTlvValueStartEndOffsets(aRxInfo.mMessage, Tlv::kAddressRegistration, offset, endOffset));