windows/msvc: Introduce an msbuild variable for third-party sources.

This is consistent with extmod.mk's split between SRC_EXTMOD_C and
SRC_THIRDPARTY_C, the latter specifying source files which get
compiled but not included in qstr generation.

Signed-off-by: stijn <stijn@ignitron.net>

ports/windows/micropython.vcxproj

@@ -87,6 +87,7 @@
   <ItemGroup>
     <ClCompile Include="@(PyCoreSource)" />
     <ClCompile Include="@(PyExtModSource)" />
+    <ClCompile Include="@(PyThirdPartySource)" />
     <ClCompile Include="$(PyBaseDir)shared\readline\*.c" />
     <ClCompile Include="$(PyBaseDir)shared\runtime\gchelper_generic.c" />
     <ClCompile Include="$(PyBaseDir)ports\windows\*.c" />

ports/windows/msvc/genhdr.targets

@@ -66,7 +66,7 @@ using(var outFile = System.IO.File.CreateText(OutputFile)) {
     <ItemGroup>
       <PyIncDirs Include="$(PyIncDirs)"/>
       <PreProcDefs Include="%(ClCompile.PreProcessorDefinitions);NO_QSTR"/>
-      <PyQstrSourceFiles Include="@(ClCompile)" Exclude="$(PyBuildDir)\frozen_content.c">
+      <PyQstrSourceFiles Include="@(ClCompile)" Exclude="$(PyBuildDir)\frozen_content.c;@(PyThirdPartySource)">
         <Changed>False</Changed>
         <OutFile>$([System.String]::new('%(FullPath)').Replace('$(PyBaseDir)', '$(DestDir)qstr\'))</OutFile>
       </PyQstrSourceFiles>