libev: don't install event.h

It's a 'compatibility layer' with libevent which we:
1) Don't need
2) Possibly outdated with respect to libevent2
3) Causes build failures

Fixes:
http://autobuild.buildroot.net/results/451/4510fe02ef3497803ed27bf339dca07b3b073c10/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>