Now that lz4 kernel compression is available, add *.lz4 to .gitignore.

Signed-off-by: Markus Trippelsdorf <[email protected]>
Acked-by: Kyungsik Lee <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>