Add NETStandard.Library to CoreCLR.Eventing

As a bottom-level dependency, it always needed it. The alternative is
hunting down every particular dependency, but that's what the
meta-package is useful for.

src/Microsoft.PowerShell.CoreCLR.Eventing/project.json

@@ -17,6 +17,7 @@
                 "define": [ "CORECLR" ]
             },
             "dependencies": {
+                "NETStandard.Library": "1.6.0",
                 "System.Security.Principal": "4.0.1",
                 "System.Security.Principal.Windows": "4.0.0",
                 "System.Diagnostics.TraceSource": "4.0.0",