scripts/gdb: add constants.py to .gitignore

Since scripts/gdb/linux/constants.py is autogenerated, this should have
been added to .gitignore when it was introduced.

Fixes: f197d75f ("scripts/gdb: provide linux constants")
Link: http://lkml.kernel.org/r/1467127337-11135-4-git-send-email-kieran@bingham.xyz


Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Kieran Bingham <kieran@bingham.xyz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>