databases/mysql-proxy: unbreak with libevent >= 2.1

chassis-mainloop.c:102:54: error: use of undeclared identifier '_EVENT_VERSION'

if (0 != chassis_check_version(event_get_version(), _EVENT_VERSION)) {
                                                    ^

chassis-mainloop.c:104:15: error: use of undeclared identifier '_EVENT_VERSION'

G_STRLOC, _EVENT_VERSION, event_get_version());
          ^

/usr/local/include/glib-2.0/glib/gmessages.h:161:32: note: expanded from macro 'g_critical'

__VA_ARGS__)
^

chassis-mainloop.c:119:37: error: use of undeclared identifier '_EVENT_VERSION'

chas->event_hdr_version = g_strdup(_EVENT_VERSION);
                                   ^

PR: 216527