Add void to functions with no arguments to prevent compiler warning

[dundargoc]

Clang 15.0.0 will give the following warning if (void) isn't provided:

warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]

[xiaocang]

already merged here: https://github.com/openresty/lua-cjson/commit/1dff61d9bf390372124c13443e0131a96383f5bc