Guard pk_sign_ext with PSA_CRYPTO_C

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 2ef3b91..45c80b4 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -535,6 +535,7 @@
              unsigned char *sig, size_t sig_size, size_t *sig_len,
              int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
 
+#if defined(MBEDTLS_PSA_CRYPTO_C)
 /**
  * \brief           Make signature with input pk type. not the type of \p ctx .
  *
@@ -574,6 +575,7 @@
                          unsigned char *sig, size_t sig_size, size_t *sig_len,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng );
+#endif /* MBEDTLS_PSA_CRYPTO_C */
 
 /**
  * \brief           Restartable version of \c mbedtls_pk_sign()