supertux: select BR2_PACKAGE_LIBCURL

libcurl is unconditionally added to the dependencies list in
supertux.mk. However libcurl is not selected in Config.in.

This patch fixes that.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Acked-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>