buildroot

package/minetest: add libcurl optional dependency

Curl support is strongly recommended for multiplayer game.

The build system warn when curl is disabled:
  cURL is required to load the server list
  cURL is required to announce to the server list

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>