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>

configs/rock64_defconfig

@@ -26,6 +26,7 @@ BR2_TARGET_UBOOT_CUSTOM_REPO_URL="https://github.com/ayufan-rock64/linux-u-boot.
 BR2_TARGET_UBOOT_CUSTOM_REPO_VERSION="2017.09-rockchip-ayufan-1035-gd646df03ac"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="rock64-rk3328"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
+BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y
 BR2_TARGET_UBOOT_NEEDS_ATF_BL31=y
 BR2_TARGET_UBOOT_NEEDS_ATF_BL31_ELF=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM=y