Add const to serial argument in mbedtls_x509write_crt_set_serial_raw

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 1f8a006..663b308 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -94,7 +94,7 @@
 }
 
 int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
-                                         unsigned char *serial, size_t serial_len)
+                                         const unsigned char *serial, size_t serial_len)
 {
     if (serial_len > MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN) {
         return MBEDTLS_ERR_X509_BAD_INPUT_DATA;