libevent: Bump to 2.0.21

This fixes the build on ARC as well, as 2.0.17+ includes a fix
that checks for existence of 'sysctl'.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libevent/libevent.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-LIBEVENT_VERSION = 2.0.14
+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