configs/targets/nios2-softmmu: Add TARGET_NEED_FDT=y to the nios2 config

qemu-system-nios2 uses the functions from libfdt in hw/nios2/boot.c,
so this target has to be marked with TARGET_NEED_FDT=y in its config
file.

Message-Id: <20230119125745.2028814-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configs/targets/nios2-softmmu.mak

 TARGET_ARCH=nios2
 TARGET_ALIGNED_ONLY=y
+TARGET_NEED_FDT=y