codegen: only touch wocky-signals-marshal.list if it's actually changed
codegen: only touch wocky-signals-marshal.list if it's actually changed

Previously, if you edit any file in $(HANDWRITTEN_SOURCES), then
wocky-signals-marshal.list would be recreated, meaning
wocky-signals-marshal.{c,h} would also be recreated, meaning half of
wocky would be rebuilt. This is not cool.

Signed-off-by: Jonny Lamb <3d1fb3642aaab2ff70e3392a5ab916bf99eed6a3@collabora.co.uk>

noonien-d/wocky,noonien-d/wocky,noonien-d/wocky,freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky