Ninja: Restore detection of msvc-wine showIncludes prefix

Since !8129 (merged) 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 (closed)
Backport: release