Declare an empty _CollectWatchItems target

Fixes #732

Author: AArnott

src/Nerdbank.GitVersioning.Tasks/build/PrivateP2PCaching.proj

@@ -25,6 +25,9 @@
 
   <!-- WinUI3 seems to want this target -->
   <Target Name="GetTargetPathWithTargetPlatformMoniker" />
+  
+  <!-- dotnet watch needs this target -->
+  <Target Name="_CollectWatchItems" />
 
   <Import Project="$(NBGV_PrivateP2PAuxTargets)" Condition=" '$(NBGV_PrivateP2PAuxTargets)' != '' " />
 </Project>