BUG/MINOR: hlua: hlua_yieldk ctx argument should support pointers

BUG/MINOR: hlua: hlua_yieldk ctx argument should support pointers

lua_yieldk ctx argument is of type lua_KContext which is typedefed to
intptr_t when available so it can be used to store pointers.

But the wrapper function hlua_yieldk() passes it as a regular it so it
breaks that promise.

Changing hlua_yieldk() prototype so that ctx argument is of type
lua_KContext.

This bug had no functional impact because ctx argument is not being
actively used so far. This may be backported to all stable versions
anyway.