Barebox-imd is also used in the PBL. As the object file names are different
for the PBL, the explicit dependency on generated/compile.h wasn't there.
This leads to random build failures in parallel builds, or worse the PBL
picking up the old compile.h defines from an earlier build.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>