Put headers inside ifdef _AIX.

Prevents compile errors due to missing definitions (eg va_list) on
non-AIX platforms.

openbsd-compat/port-aix.c

@@ -26,6 +26,8 @@
  */
 #include "includes.h"
 
+#ifdef _AIX
+
 #include "xmalloc.h"
 #include "sshbuf.h"
 #include "ssherr.h"
@@ -36,8 +38,6 @@
 #include "ssh_api.h"
 #include "log.h"
 
-#ifdef _AIX
-
 #include <errno.h>
 #if defined(HAVE_NETDB_H)
 # include <netdb.h>