scripts/gdb: silence 'nothing to do' message

The constants.py generation, involves a rule to link into the main
makefile.  This rule has no command and generates a spurious warning
message in the build logs when CONFIG_SCRIPTS_GDB is enabled.

Fix simply by giving a no-op action

Link: http://lkml.kernel.org/r/1467127337-11135-2-git-send-email-kieran@bingham.xyz


Signed-off-by: Kieran Bingham <kieran@bingham.xyz>
Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>