openssh: -fno-common fix from upstream f47d72ddad

This is currently staged in vendor/ as part of the 8.0p1 import, which isn't
quite ready to land. Given that this is a simple one-line fix, apply it now
as the fallout will be pretty minimal.

-fno-common will become the default in GCC10/LLVM11.

MFC after:	3 days

crypto/openssh/session.c

@@ -143,7 +143,7 @@ extern int startup_pipe;
 extern void destroy_sensitive_data(void);
 extern struct sshbuf *loginmsg;
 extern struct sshauthopt *auth_opts;
-char *tun_fwd_ifnames; /* serverloop.c */
+extern char *tun_fwd_ifnames; /* serverloop.c */
 
 /* original command from peer. */
 const char *original_command = NULL;