Ninja: Fix clang-cl /showIncludes prefix detection

We run MSVC-like compilers with the `/showIncludes` option and match the
output to extract the corresponding message.  Fix the matching to
support compilers like `clang-cl` that print the message on the first
line such that it is not preceded by a newline.

LLVM-Issue: https://llvm.org/bugs/show_bug.cgi?id=27226