configs: orangepipc defconfig needs dtc for u-boot

As can be seen at
https://travis-ci.org/buildroot/buildroot-defconfig-testing/jobs/99518455

,
the new Orange Pi PC defconfig fails to build due to DTC being missing
when building U-Boot. This commit adds the appropriate option to add
DTC as a dependency to U-Boot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>