Export CBS_asn1_ber_to_der.

We don't really want to expose this but the bytestring test calls this
function directly and, when linked against a shared library, thus needs
it to be in the dynamic symbol table.

Change-Id: Ife01da36fe142026a6a3e545cff746512b966ee6

crypto/bytestring/internal.h

diff --git a/crypto/bytestring/internal.h b/crypto/bytestring/internal.h
index 42c1a39..fd46aeb 100644
--- a/crypto/bytestring/internal.h
+++ b/crypto/bytestring/internal.h
@@ -45,7 +45,7 @@
  * from NSS.
  *
  * It returns one on success and zero otherwise. */
-int CBS_asn1_ber_to_der(CBS *in, uint8_t **out, size_t *out_len);
+OPENSSL_EXPORT int CBS_asn1_ber_to_der(CBS *in, uint8_t **out, size_t *out_len);
 
 
 #if defined(__cplusplus)