decrease required go version to 1.12

[oleg-jukovec]

The error occurs when trying to build the library using go < 1.16:
./alloc.go:14:10: unrecognized Go type _Ctype_void
./alloc.go:16:21: unrecognized Go type _Ctype_void

This is an unnecessary restriction. It can be fixed by replacing
*C.void with unsafe.Pointer (as it is already done for ptr argument of
CRYPTO_EX_free[1][2]).

1. https://www.openssl.org/docs/manmaster/man3/SSL_get_ex_new_index.html
2. https://www.openssl.org/docs/man3.0/man3/CRYPTO_EX_free.html

Related to #18

Motivation:

I want to use the library with go 1.13.

Before the fix:

$ go version
go version go1.12.17 linux/amd64
$ go test
# github.com/libp2p/go-openssl
./alloc.go:14:10: unrecognized Go type _Ctype_void
./alloc.go:16:21: unrecognized Go type _Ctype_void
./alloc.go:14:10: unrecognized Go type _Ctype_void
./alloc.go:16:21: unrecognized Go type _Ctype_void
FAIL	github.com/libp2p/go-openssl [build failed]

After the fix:

$ go version
go version go1.12.17 linux/amd64
$ go test .
ok  	github.com/oleg-jukovec/go-openssl	4.333s

[MarcoPolo]

Can you use Go 1.17 or 1.18 instead? We, like the Go team with its own security policies, only support the last two major releases of Go.

Closing this as wontfix, but thanks for figuring out a workaround for folks who may find this in the future.