Un-ignore "vmlinux.lds.S" in .gitignore

We ignore all the generated files called "vmlinux*" from the top-level
gitignore, but that also ends up catching a few files that we track, and
that people do edit.  Notably the "vmlinux.lds.S" file, that each
architecture has.

You can always use "git add -f" to override the ignore file, but we
might as well just make it explicit for this file.

Bitten-by-and-tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>