Include ssh_api.h for struct ssh.

struct ssh is needed by implementations of sys_auth_passwd() that were
converted in commit bba02a50.  Needed to fix build on AIX, I assume for
the other platforms too (although it should be harmless if not needed).

auth-sia.c

@@ -36,6 +36,7 @@
 #include <string.h>
 
 #include "ssh.h"
+#include "ssh_api.h"
 #include "key.h"
 #include "hostfile.h"
 #include "auth.h"

openbsd-compat/port-aix.c

@@ -32,6 +32,7 @@
 #include "hostfile.h"
 #include "auth.h"
 #include "ssh.h"
+#include "ssh_api.h"
 #include "log.h"
 
 #ifdef _AIX

openbsd-compat/port-uw.c

@@ -47,6 +47,7 @@
 #include "hostfile.h"
 #include "auth.h"
 #include "ssh.h"
+#include "ssh_api.h"
 
 int nischeck(char *);