Fix debuginfod build failure when httplib is used. (#156107)
This is a follow up of adbd43250ade1d5357542d8bd7c3dfed212ddec0. The
problem is, HTTPServer class will lost its implicit move ctor if httplib
is used. This patch adds the move ctor explicitly to solve this issue.
The default move ctor is not added in the header due to a limitation
that "httplib::Server" is a forward declaration and it is incomplete.