Remove pointer cast in mbedtls_x509_oid_get_sig_alg

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index e1d5758..1f8a006 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -416,7 +416,7 @@
         return MBEDTLS_ERR_X509_INVALID_ALG;
     }
 
-    if ((ret = mbedtls_x509_oid_get_oid_by_sig_alg(pk_alg, ctx->md_alg,
+    if ((ret = mbedtls_x509_oid_get_oid_by_sig_alg((mbedtls_pk_sigalg_t) pk_alg, ctx->md_alg,
                                                    &sig_oid, &sig_oid_len)) != 0) {
         return ret;
     }