Ninja: Restore detection of clang-cl 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 `clang-cl`, which uses a relative path, forward
slashes, and is always in English [1]:

    Note: including file: ./foo.h

Update the regex to match that too.

[1] https://github.com/llvm/llvm-project/blob/llvmorg-16.0.0/clang/lib/Frontend/HeaderIncludeGen.cpp#L102

Co-authored-by: Brad King <brad.king@kitware.com>