Unignore "packages/*/build" in VisualStudio.gitignore.

VisualStudio.gitignore

@@ -15,6 +15,9 @@ build/
 [Bb]in/
 [Oo]bj/
 
+# Enable "build/" folder in the NuGet Packages folder since NuGet packages use it for MSBuild targets
+!packages/*/build/
+
 # MSTest test Results
 [Tt]est[Rr]esult*/
 [Bb]uild[Ll]og.*