configs/hifive_unleashed_defconfig: uboot needs pylibfdt

uboot needs Python libfdt to build:

  pylibfdt does not seem to be available with python3

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4839060137

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/hifive_unleashed_defconfig

@@ -48,6 +48,7 @@ BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="sifive_unleashed"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
 BR2_TARGET_UBOOT_NEEDS_OPENSBI=y
+BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot.itb"
 BR2_TARGET_UBOOT_SPL=y