Ninja: Restore detection of msvc-wine showIncludes prefix

Since commit 8f82e755f3 (Ninja: Fix detection of MSVC showIncludes
prefix in Italian, 2023-01-26, v3.26.0-rc1~20^2) our regex no longer
matches the output from `msvc-wine`, which uses forward slashes:

    Note: including file: /path/to/foo.h

`cl /showIncludes` under Wine prints paths of the form `Z:\path\to\file`,
but the `msvc-wine` wrapper converts them to the form `/path/to/file` so
that native Ninja can be used.  Update our regex to match the prefix
followed by a path with a leading forward slash.

Fixes: #24908