[Fix] Use correct type for keylen in lua_ucl_newindex

[Fix] Use correct type for keylen in lua_ucl_newindex

The keylen variable used in lua_ucl_newindex function should use size_t
type instead of lua_Integer, because all functions that use keylen
expect it to be of size_t type. This mismatch leads to incompatible
pointer types, and modern versions of GCC fail to compile the code.

Fixes: 9e87597ceb05 ("[Project] Allow manipulations with opaque UCL objects")
Issue: https://github.com/rspamd/rspamd/issues/5163