seafile-server uses libevhtp pthreads support.  libevhtp needs
libevent2-pthreads for that. However, since there is no installable
package for libevhtp--the library is build static-only--then
seafile-server, its consumer, needs to add the dependecy.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>