crypto/bio/bss_dgram.c: annotate fallthrough (-Wimplicit-fallthrough)

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4916)