package/libwebsockets: fix build with libev and libevent

package/libwebsockets: fix build with libev and libevent

libev and libevent can't be enabled at the same time since version 3.0.0
and
https://github.com/warmcat/libwebsockets/commit/ff2f5f601d5965f5860bc989d05b61246f172095

So prefer libev over libevent to have the same default behavior than
upstream LWS_WITH_DISTRO_RECOMMENDED target

Fixes:
 - http://autobuild.buildroot.org/results/ee5ef79a39570bcf5c605b1a4b6b48523605d647

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>