Fix compile error when registering new user types with operator()

[matusfedorko]

Compiling with the latest develop branch, the following would fail to compile (tested in MSVC 2019):

#define SOL_ALL_SAFETIES_ON 1
#include <sol/sol.hpp>

struct Ptr
{
	int* operator()() { return nullptr; }
	const int* operator()() const { return nullptr; }
};

int main()
{
	sol::state lua;
	lua.new_usertype<Ptr>("Ptr");
	return 0;
}

[ThePhD]

I think this should work, but I need to run the tests for a bunch of platforms to make sure.

I also need to get GitHub Actions running and figure out how to rip these appveyor/travis-ci checks out of the PRs already...

[ThePhD]

We're about to find out how broken this is! Thanks for submitting.