build_helpers: Fix TC3 default DPE test

The default Attestation scheme for TC3 is DPE but a
reference to tf-m-extras is missing to actually run the
DPE test.

Add it with this patch.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I798bab34dd2f03347eaad98437cb116c3d32b67a
2 files changed