windows: Make project file read-only for IDE.

Avoids the 'warning: Wildcards in project items are not supported'
message from the C++ project system in Visual Studio, while otherwise
remaining completely functional.

ports/windows/micropython.vcxproj

@@ -21,6 +21,7 @@
   <PropertyGroup Label="Globals">
     <ProjectGuid>{740F3C30-EB6C-4B59-9C50-AE4D5A4A9D12}</ProjectGuid>
     <RootNamespace>micropython</RootNamespace>
+    <ReadOnlyProject>true</ReadOnlyProject>
   </PropertyGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">