ssl_test_lib: add guards for pk_wrap_as_opaque()

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/programs/ssl/ssl_test_lib.h b/programs/ssl/ssl_test_lib.h
index 5cb6a36..1da2dfb 100644
--- a/programs/ssl/ssl_test_lib.h
+++ b/programs/ssl/ssl_test_lib.h
@@ -236,6 +236,7 @@
                              psa_key_usage_t *usage,
                              mbedtls_pk_type_t key_type);
 
+#if defined(MBEDTLS_PK_C)
 /** Turn a non-opaque PK context into an opaque one with folowing steps:
  * - extract the key data and attributes from the PK context.
  * - import the key material into PSA.
@@ -258,6 +259,7 @@
  */
 int pk_wrap_as_opaque(mbedtls_pk_context *pk, psa_algorithm_t psa_alg, psa_algorithm_t psa_alg2,
                       psa_key_usage_t psa_usage, mbedtls_svc_key_id_t *key_id);
+#endif /* MBEDTLS_PK_C */
 #endif /* MBEDTLS_USE_PSA_CRYPTO */
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO) && defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)