libmicrohttpd: disable spdy support

It requires OpenSSL and IPv6 support and it should probably be an
option since SSL support uses GnuTLS thus making a huge target
footprint with 2 SSL/TLS libraries for full support.
But most of all it's considered experimental.
Fixes:
http://autobuild.buildroot.net/results/836/836c39c11cecbcf9399675ba3ab01202417b9642/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>