Fix scope for GenerateWeaveNodeId

When compiling the ios build internally, we see the below error, add
::nl:Weave scope can resolve this issue.

OBJCXX libNLWeaveDeviceManager_a-NLWeaveKeyIds.o

/src/device-manager/cocoa/NLWeaveStack.mm:197:11: error: use of
undeclared identifier 'GenerateWeaveNodeId'; did you mean
'nl::Weave::GenerateWeaveNodeId'?
err = GenerateWeaveNodeId(_mFabricState.LocalNodeId);
      ^~~~~~~~~~~~~~~~~~~
 nl::Weave::GenerateWeaveNodeId
 ../../../src/include/Weave/Core/WeaveMessageLayer.h:1076:20:
note: 'nl::Weave::GenerateWeaveNodeId' declared here
extern WEAVE_ERROR GenerateWeaveNodeId(uint64_t & nodeId);
diff --git a/src/device-manager/cocoa/NLWeaveStack.mm b/src/device-manager/cocoa/NLWeaveStack.mm
index c98329f..e9345b2 100644
--- a/src/device-manager/cocoa/NLWeaveStack.mm
+++ b/src/device-manager/cocoa/NLWeaveStack.mm
@@ -194,7 +194,7 @@
     _mFabricState.FabricId = 0;
 
     // Generate a unique node id for local Weave stack.
-    err = GenerateWeaveNodeId(_mFabricState.LocalNodeId);
+    err = ::nl::Weave::GenerateWeaveNodeId(_mFabricState.LocalNodeId);
     SuccessOrExit(err);
 
     // Configure the weave listening address, if one was provided