bpo-37333: Ensure IncludeTkinter has a value (GH-14240)

diff --git a/PCbuild/python.props b/PCbuild/python.props
index 0f93a00..c38be2c 100644
--- a/PCbuild/python.props
+++ b/PCbuild/python.props
@@ -81,6 +81,9 @@
     
     <!-- Full path of the resulting python.exe binary -->
     <PythonExe Condition="'$(PythonExe)' == ''">$(BuildPath)python$(PyDebugExt).exe</PythonExe>
+
+    <!-- Include Tkinter by default -->
+    <IncludeTkinter Condition="'$(IncludeTkinter)' == ''">true</IncludeTkinter>
   </PropertyGroup>
   
   <PropertyGroup Condition="'$(Platform)'=='ARM'" Label="ArmConfiguration">