rgw: rgw_admin also inits curl

rgw: rgw_admin also inits curl

since we use http manager which in turn uses curl and uses curl multi
interfaces. While curl is initialized at the first call of curl_easy_init() this
method isn't guaranteed to be safe when multiple threads may call the function
since curl_global_init isn't reentrant. Calling curl_global_init via
rgw::curl::setup_curl which additionally sets up ssl interfaces etc. when
openssl is used as curl's ssl backend. Similarly moving rgw target link to
accomodate this change.

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>