[CMAKE] Always build obj/build.h

Summary:
The file needs to be always generated.
This requires the use of a custom_target (always out of date) rather
than a custom_command.

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja

Check the buildcmake/obj/build.h file content matches your current
commit hash.
Amend the commit message then run ninja again: the
buildcmake/obj/build.h should be updated accordingly.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4111