Skin: Suppress warning regarding use of wildcards in project files

Skin projects are created just once and then they are not supposed to be
changed (from within VS).
So we can mark them as _read only_ projects.

https://learn.microsoft.com/en-us/cpp/build/reference/vcxproj-files-and-wildcards?view=msvc-170

Src/Skins/Skin.props

@@ -5,6 +5,7 @@
     <IntDir>$(Configuration)\</IntDir>
     <IgnoreImportLibrary>true</IgnoreImportLibrary>
     <LinkIncremental>false</LinkIncremental>
+    <ReadOnlyProject>true</ReadOnlyProject>
   </PropertyGroup>
   <ItemDefinitionGroup>
     <Link>