Export |CBS_get_any_ber_asn1_element|.

We have need of it internally.

Change-Id: I564af468728b22245e8eab384ea7018b7e88cc86
Reviewed-on: https://boringssl-review.googlesource.com/5022
Reviewed-by: Matt Braithwaite <mab@google.com>
Reviewed-by: Adam Langley <agl@google.com>