s390/decompressor: Ignore file vmlinux.bin.full

Commit 81796a3c ("s390/decompressor: trim uncompressed
image head during the build") introduced a new
file named vmlinux.bin.full in directory
arch/s390/boot/compressed.

Add this file to the list of ignored files so it does
not show up on git status.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>