Supply missing header guards

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190604181618.19980-5-armbru@redhat.com>
diff --git a/linux-user/target_flat.h b/linux-user/target_flat.h
index 0ba6bdd..8fe189e 100644
--- a/linux-user/target_flat.h
+++ b/linux-user/target_flat.h
@@ -1,6 +1,10 @@
 /* If your arch needs to do custom stuff, create your own target_flat.h
  * header file in linux-user/<your arch>/
  */
+
+#ifndef LINUX_USER_TARGET_FLAT_H
+#define LINUX_USER_TARGET_FLAT_H
+
 #define flat_argvp_envp_on_stack()                           1
 #define flat_reloc_valid(reloc, size)                        ((reloc) <= (size))
 #define flat_old_ram_flag(flag)                              (flag)
@@ -8,3 +12,5 @@
 #define flat_get_addr_from_rp(rp, relval, flags, persistent) (rp)
 #define flat_set_persistent(relval, persistent)              (*persistent)
 #define flat_put_addr_at_rp(rp, addr, relval)                put_user_ual(addr, rp)
+
+#endif