riscv: requires libfdt

When compiling on a machine without libfdt installed the configure script
should try to get libfdt from the git or should die because otherwise
CONFIG_LIBFDT is not set and the build process end in an error in the link
phase.. eg:

hw/riscv/virt.o: In function `riscv_virt_board_init':
qemu/src/hw/riscv/virt.c:317: undefined reference to `qemu_fdt_setprop_cell'
qemu/src/hw/riscv/virt.c:319: undefined reference to `qemu_fdt_setprop_cell'
qemu/src/hw/riscv/virt.c:345: undefined reference to `qemu_fdt_dumpdtb'
collect2: error: ld returned 1 exit status
make[1]: *** [qemu-system-riscv64] Error 1
make: *** [subdir-riscv64-softmmu] Error 2

Cc: qemu-stable@nongnu.org
Reviewed-by: NBastian Koppelmann <kbastian@mail.uni-paderborn.de>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NMichael Clark <mjc@sifive.com>
Signed-off-by: NKONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: NMichael Clark <mjc@sifive.com>

Message-Id: <1525360636-18229-4-git-send-email-frederic.konrad@adacore.com>

configure

@@ -3761,7 +3761,7 @@ fi
 fdt_required=no
 for target in $target_list; do
   case $target in
-    aarch64*-softmmu|arm*-softmmu|ppc*-softmmu|microblaze*-softmmu|mips64el-softmmu)
+    aarch64*-softmmu|arm*-softmmu|ppc*-softmmu|microblaze*-softmmu|mips64el-softmmu|riscv*-softmmu)
       fdt_required=yes
     ;;
   esac