.gitignore: ignore vmlinuz

MIPS compressed kernels output a vmlinuz file in the top-level directory
(maybe others do).  Add vmlinuz to the list of files to ignore by git.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>