libmemcached: disable libmudflapth support

We're not handling it (copying it) for external toolchains and it's
causing build failures. Fixes:
http://autobuild.buildroot.net/results/9d1/9d1e414850307487d65435216ae0ddee56d1098f/
http://autobuild.buildroot.net/results/f08/f08c8e5514e054397428677f08fadac5386189dc/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>