gitignore usr/initramfs_data.cpio.bz2 and usr/initramfs_data.cpio.lzma

usr/initramfs_data.cpio.bz2 and usr/initramfs_data.cpio.lzma are binary
files should be ignored

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>