s390/boot: Ignore vmlinux.map

When building with CONFIG_VMLINUX_MAP=y, a decompressor vmlinux.map file
is generated in the boot directory.

Add this file to .gitignore to ensure Git does not track it.

Signed-off-by: WangYuli <wangyuli@uniontech.com>
Link: https://lore.kernel.org/r/F884C733016D6715+20250311030824.675683-1-wangyuli@uniontech.com


Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>