.gitignore: stop ignoring `.manifest` files

On Windows, it is possible to embed additional metadata into an executable by linking in a "manifest", i.e. an XML document that describes capabilities and requirements (such as minimum or maximum Windows version). These XML documents are expected to be stored in `.manifest` files. At least _some_ Visual Studio versions auto-generate `.manifest` files when none is specified explicitly, therefore we used to ask Git to ignore them. However, we do have a beautiful `.manifest` file now: `compat/win32/git.manifest`, so neither does Visual Studio auto-generate a manifest for us, nor do we want Git to ignore the `.manifest` files anymore. Further reading on auto-generated `.manifest` files: https://docs.microsoft.com/en-us/cpp/build/manifest-generation-in-visual-studio Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>