Changed type to get libmemcached >= 1.0.17 to compile. Still gives warning ...

Potential fix for stevedekorte/io#247 and Homebrew/homebrew#25315.

I think some cpp stuff needs to be done here to account for older and younger versions of libmemcached, but I'm not sure how that all works.

Still gives warning for:
io/addons/Memcached/source/IoMemcached.c:46:2: warning: implicit declaration of function IoState_registerProtoWithFunc_ is invalid in C99