libwebsockets: needs host-openssl when built with openssl support

Fixes
http://autobuild.buildroot.net/results/d01/d0190a6e5c6b5aa78cfefc29e4e8a4e4b135450b/
http://autobuild.buildroot.net/results/9bb/9bb8defedb8024de83eb3a609ab584efb5d0d2b0/
http://autobuild.buildroot.net/results/e4a/e4a1b298e7e158d3614b89e61d40ef9a00e7ff67/
http://autobuild.buildroot.net/results/5e2/5e24a90058111f7d5618047410f809b45bc165f6/



And many others.

The build system wants to create a ssl key for testing using host-openssl,
so ensure it is available.

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