libmicrohttpd: disable curl testcases

They require a toolchain with IPv6. Fixes:
http://autobuild.buildroot.net/results/edb/edb1b5ee1ffaa6a495b4d7c2976f618c8331b5d2/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>