libevent: fix openssl detection when statically linking

Fixes (indirectly):
http://autobuild.buildroot.net/results/dbf/dbf2d348de3831a86728f9d72cb56b9f2ddbf554/
http://autobuild.buildroot.net/results/6a8/6a89183e558e4ff27841c3e544687ad4a04fb036/
http://autobuild.buildroot.net/results/c80/c80eedb4d7335f1aef736a1312900c2b371eabc9/
http://autobuild.buildroot.net/results/91b/91b4ab54ed9d422bfaef765d09cc92192012ce7e/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>