Add DTLS to SSL_get_version

(Imported from upstream's 504e643e09)

Change-Id: I94f4fea105b6eeb001d376ca3ffef77d1432aa88
Reviewed-on: https://boringssl-review.googlesource.com/4462
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_lib.c

@@ -2350,6 +2350,12 @@ static const char *ssl_get_version(int version) {
     case SSL3_VERSION:
       return "SSLv3";
 
+    case DTLS1_VERSION:
+      return "DTLSv1";
+
+    case DTLS1_2_VERSION:
+      return "DTLSv1.2";
+
     default:
       return "unknown";
   }