switch to mbedtls_pk_sigalg_t

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 09c2328..93cdd2c 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -587,7 +587,7 @@
     c2 = buf + size;
     MBEDTLS_ASN1_CHK_ADD(sig_and_oid_len, mbedtls_x509_write_sig(&c2, c,
                                                                  sig_oid, sig_oid_len,
-                                                                 sig, sig_len, pk_alg));
+                                                                 sig, sig_len, (mbedtls_pk_sigalg_t)pk_alg));
 
     /*
      * Memory layout after this step: