configs/rock64: needs U-Boot pylibfdt

The build currently fails with:

*** dtoc needs the Python libfdt library. Either
*** install it on your system, or try:
***
*** sudo apt-get install swig libpython-dev
***
*** to have U-Boot build its own version.

Adding BR2_TARGET_UBOOT_NEEDS_PYLIBFDT should fix this build issue,
which was reported at:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/158295223



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>