socrates_cyclone5_defconfig: U-Boot needs DTC

Probably not noticed before because dtc was already built or it was
installed on the host.

Fixes: https://gitlab.com/arnout/buildroot/-/jobs/28979369



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Michał Łyszczek <michal.lyszczek@bofc.pl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>