Guard missing in nuget

Author: SimonCropp

src/Polyfill/Polyfill.nuspec

@@ -17,6 +17,8 @@
     <file src="../icon.png" target="icon.png" />
     <file src="*.cs"
           target="contentFiles/cs/netstandard2.0/Polyfill/"/>
+    <file src="Guard\*.cs"
+          target="contentFiles/cs/netstandard2.0/Polyfill/Guard"/>
     <file src="Nullable\*.cs"
           target="contentFiles/cs/netstandard2.0/Polyfill/Nullable"/>
     <file src="Nullability\*.cs"