[IA64] Add gate.lds to list of files ignored by Git

If you build the kernel `in-place' then do a git update, git
complains about arch/ia64/kernel/gate.lds being modified and
untracked.

Add that (generated) file to a .gitignore file.

Signed-off-by: Peter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>