.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.

---
 .gitignore |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 8faa6c0..5d56a3f 100644
@@ -28,6 +28,7 @@ modules.builtin
 *.gz
 *.bz2
 *.lzma
+*.xz
 *.lzo
 *.patch
 *.gcno

1.7.4.1