Include <errno.h> for global errno

At least on OpenBSD the symbol `errno` is otherwise not defined;  it is
used in the `SSO()` macro and `send_authorized_keys()` function.

tmate-session.c

@@ -5,6 +5,7 @@
 #include <netinet/in.h>
 #include <sys/socket.h>
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

tmate-ssh-client.c

@@ -1,6 +1,7 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <netinet/tcp.h>
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <event.h>