Patch for GC.compact memory issue

[Ngan Pham <nganpham@gmail.com>]

Hello, I'd like to submit a patch to fix an issue after running
`GC.compact`. The LOCALHOST variable is defined in Ruby and is
resolved and memoized (to `localhost`) in C once. However, the pointer
in C is not GC marked. Running `GC.compact` potentially moves the
`LOCALHOST` object in Ruby causing `localhost` to point to a bad or
empty slot. This causes `env['REMOTE_ADDR']` to  a bad object in
Unicorn.

Some useful links:
- `GC.compact` introduced: https://bugs.ruby-lang.org/issues/15626.
- My conversation with tenderlove over twitter on this issue:
https://twitter.com/tenderlove/status/1396625048861954049

Thanks,
Ngan

diff --git a/ext/kgio/accept.c b/ext/kgio/accept.c
index f1de39d..5a5f15a 100644
--- a/ext/kgio/accept.c
+++ b/ext/kgio/accept.c
@@ -497,7 +497,14 @@ void init_kgio_accept(void)
  */
  rb_define_const(mKgio, "SOCK_CLOEXEC", INT2NUM(SOCK_CLOEXEC));

+ /*
+ * Resolve LOCALHOST from Ruby land and memoize it for fast lookup.
+ * We have to GC mark the object since it's defined in Ruby and we
+ * don't want GC.compact to move it.
+ */
  localhost = rb_const_get(mKgio, rb_intern("LOCALHOST"));
+ rb_gc_register_mark_object(localhost);
+
  cKgio_Socket = rb_const_get(mKgio, rb_intern("Socket"));
  cClientSocket = cKgio_Socket;
  mSocketMethods = rb_const_get(mKgio, rb_intern("SocketMethods"));