web_service/CMakeLists: Make the CPPHTTPLIB_OPENSSL_SUPPORT constrained to the...

web_service/CMakeLists: Make the CPPHTTPLIB_OPENSSL_SUPPORT constrained to the web_service library only

Given we link in httplib privately, we can also make the definition
enabling OpenSSL support private as well. Prevents leaking a definition
into other libraries that link with this one, like the core library.