.gitignore: ignore *.xz files

Building with CONFIG_KERNEL_XZ results in the following:

 # Untracked files:
 #   (use "git add <file>..." to include in what will be committed)
 #
 #       arch/x86/boot/compressed/vmlinux.bin.xz

So ignore xz-compressed files at the top level like we already do for
other compression types.

Signed-off-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>