bpo-35134: Add cpython/pymem.h to build system (GH-12842)

diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index a980799..a135e93 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -117,6 +117,7 @@
     <ClInclude Include="..\Include\cpython\objimpl.h" />
     <ClInclude Include="..\Include\cpython\pyerrors.h" />
     <ClInclude Include="..\Include\cpython\pylifecycle.h" />
+    <ClInclude Include="..\Include\cpython\pymem.h" />
     <ClInclude Include="..\Include\cpython\pystate.h" />
     <ClInclude Include="..\Include\cpython\tupleobject.h" />
     <ClInclude Include="..\Include\cpython\unicodeobject.h" />