scripts/gdb: rebuild constants.py on dependancy change

The autogenerated constants.py file was only being built on the initial
call, and if the constants.py.in file changed.  As we are utilising the
CPP hooks, we can successfully use the call if_changed_dep rules to
determine when to rebuild the file based on it's inclusions.

Link: http://lkml.kernel.org/r/1467127337-11135-3-git-send-email-kieran@bingham.xyz
Signed-off-by: Kieran Bingham <kieran@bingham.xyz>
Reported-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>