cmake: Fix dependency between offset.c.obj and offsets.h

The command for generating offsets.h has only been depending on the
target for offsets.c.obj, and not the file offsets.c.obj itself. Both
are needed, as explained in "CMake: dependencies between targets and
files and custom commands"[0].

This patch adds the dependency and fixes #18301

[0] https://tinyurl.com/y2hgzjhx



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>