switch to mbedtls_pk_sigalg_t

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index a3f0789..bf418a6 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -81,7 +81,7 @@
 
     mbedtls_x509_buf MBEDTLS_PRIVATE(sig);               /**< Signature: hash of the tbs part signed with the private key. */
     mbedtls_md_type_t MBEDTLS_PRIVATE(sig_md);           /**< Internal representation of the MD algorithm of the signature algorithm, e.g. MBEDTLS_MD_SHA256 */
-    mbedtls_pk_type_t MBEDTLS_PRIVATE(sig_pk);           /**< Internal representation of the Public Key algorithm of the signature algorithm, e.g. MBEDTLS_PK_RSA */
+    mbedtls_pk_sigalg_t MBEDTLS_PRIVATE(sig_pk);           /**< Internal representation of the Public Key algorithm of the signature algorithm, e.g. MBEDTLS_PK_RSA */
 
     /** Next certificate in the linked list that constitutes the CA chain.
      * \p NULL indicates the end of the list.