libevent: add license information

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libevent/libevent.mk

@@ -8,6 +8,8 @@ LIBEVENT_VERSION = 2.0.21
 LIBEVENT_SOURCE = libevent-$(LIBEVENT_VERSION)-stable.tar.gz
 LIBEVENT_SITE = https://github.com/downloads/libevent/libevent
 LIBEVENT_INSTALL_STAGING = YES
+LIBEVENT_LICENSE = BSD-3c, OpenBSD
+LIBEVENT_LICENSE_FILES = LICENSE
 
 define LIBEVENT_REMOVE_PYSCRIPT
 	rm $(TARGET_DIR)/usr/bin/event_rpcgen.py