cmake: explicitly disable openssl support for host-cmake

cmake: explicitly disable openssl support for host-cmake

host-cmake will optionally link with openssl for the embedded copy of
libarchive if available, leaking host dependencies and possibly causing
build issues in case of compatibility issues - E.G. the host-cmake version
we have in 2017.02.x doesn't build against openssl-1.1.0+:

https://github.com/libarchive/libarchive/issues/810

The openssl support in libarchive is unlikely to be needed, so explicitly
disable it.

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