Fix CS0656 in UIAutomationProvider (#9136)

Author: ericstj

src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationProvider/UIAutomationProvider.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    
+
     <EnableDefaultItems>false</EnableDefaultItems>
   </PropertyGroup>
 
@@ -52,6 +52,7 @@
     <NetCoreReference Include="Microsoft.Win32.Primitives" />
     <NetCoreReference Include="System" />
     <NetCoreReference Include="System.Drawing.Primitives" />
+    <NetCoreReference Include="System.Memory" />
     <NetCoreReference Include="System.Runtime" />
     <NetCoreReference Include="System.Resources.ResourceManager" />
     <NetCoreReference Include="System.Runtime.InteropServices" />