support/testing/tests/boot/test_atf: DTC needed for ATF Vexpress test

support/testing/tests/boot/test_atf: DTC needed for ATF Vexpress test

Since the bump of ATF to 2.2 for the ATF Vexpress test case in commit
fc3d6a3ed0f5f1ceda7ab64b8b2bee88abd29373
("support/testing/tests/boot/test_atf: update U-Boot/ATF use in
TestATFVexpress"), DTC is now needed otherwise the build fails with:

make[2]: dtc: Command not found
Makefile:873: recipe for target 'build/juno/release/fdts/juno_tb_fw_config.dtb' failed

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/674934470

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>