Overlay: ensure that the path contains the directory separator

We concatenated the path without consideration for the path separator as
the path separator was expected to be present. This is not guaranteed.
Explicitly add the missing separator which is required to construct the
proper path.

Author: compnerd

Runtimes/Overlay/Windows/clang/CMakeLists.txt

@@ -25,7 +25,7 @@ roots:
       - name: module.modulemap
         type: file
         external-contents: "@CMAKE_CURRENT_SOURCE_DIR@/ucrt.modulemap"
-  - name: "@VCToolsInstallDir@include"
+  - name: "@VCToolsInstallDir@/include"
     type: directory
     contents:
       - name: module.modulemap