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
Generalize the fix from commit 37a279f8d1 (Ninja: Write msvc_deps_prefix
as UTF-8 when console codepage is UTF-8, 2020-07-31, v3.19.0-rc1~349^2).
`cl /showIncludes` output is encoded using the console output code page,
so this is the byte sequence that Ninja must use to match its lines.
Fixes: #24068
Brad King