Avoid using anycpu for uwp libraries

Author: Arlodotexe

MultiTarget/WinUI.Extra.props

@@ -21,6 +21,10 @@
     <None PackagePath="lib/net7.0-windows10.0.18362" Include="$(MSBuildThisFileDirectory)/_._" Pack="true" />
   </ItemGroup>
 
+  <PropertyGroup Condition="'$(IsUwp)' == 'true'">
+    <Platforms>x86;x64;arm64</Platforms>
+  </PropertyGroup>
+
   <PropertyGroup Condition="'$(IsWinAppSdk)' == 'true'">
     <!-- See https://github.com/microsoft/WindowsAppSDK/issues/3842 -->
     <UseRidGraph>true</UseRidGraph>