package/unbound: use system libevent

The file descriptor limit when using the builtin mini-event
cannot handle more than 1024 file descriptors.

https://unbound.docs.nlnetlabs.nl/en/latest/topics/performance.html?highlight=libevent#using-libevent

Without explicit --with-libevent, it uses the builtin mini-event.

Signed-off-by: Kyle Harding <kyle@balena.io>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 929df7c483d19cf08badc36ef19a56735896b5d5)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/unbound/unbound.mk

@@ -17,6 +17,7 @@ UNBOUND_CONF_OPTS = \
 	--with-pidfile=/var/run/unbound.pid \
 	--with-rootkey-file=/etc/unbound/root.key \
 	--enable-tfo-server \
+	--with-libevent=$(STAGING_DIR)/usr \
 	--with-libexpat=$(STAGING_DIR)/usr \
 	--with-ssl=$(STAGING_DIR)/usr