Ninja: Update showIncludes prefix detection for clang-cl 18

Since commit LLVM/Clang commit `5523fefb01c2` ([clang][lex] Use
preferred path separator in includer-relative lookup, 2023-09-08), part
of the upcoming 18.x release, the output format of the showIncludes flag
has changed, where it now prints paths with double backslashes:

    Note: including file: .\\foo.h

Previously, we expected to see the path name in the form "./foo.h".

Extend the regex to match a path name starting with `.\`, in addition to
the existing matched patterns.