Mark unused variable in tests for cases with reduced configs

Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/tests/src/psa_exercise_key.c b/tests/src/psa_exercise_key.c
index c1e76c8..71a1b78 100644
--- a/tests/src/psa_exercise_key.c
+++ b/tests/src/psa_exercise_key.c
@@ -764,6 +764,7 @@
 #endif /* MBEDTLS_ECP_C */
 
     {
+        (void) exported;
         TEST_ASSERT( ! "Sanity check not implemented for this key type" );
     }