[IA64] Cleanup generated file not ignored by .gitignore

arch/ia64/kernel/vmlinux.lds is a generated file. Tell
git to ignore it.
Signed-off-by: NRobin Holt <holt@sgi.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>

arch/ia64/kernel/.gitignore

 gate.lds
+vmlinux.lds