autofs: fix build on Fedora 22

The following error happens when building CRIU on Fedora 22:

  CC       autofs.o
autofs.c: In function ‘autofs_mount’:
autofs.c:892:9: error: implicit declaration of function ‘add_post_prepare_cb’ [-Werror=implicit-function-declaration]
   ret = add_post_prepare_cb(autofs_add_mount_info, mi);
         ^
cc1: all warnings being treated as errors

Adding forward declaration of add_post_prepare_cb resolves the issue.
Signed-off-by: Mike Rapoport <rapoport@il.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/crtools.h

@@ -15,6 +15,7 @@
 extern int check_img_inventory(void);
 extern int write_img_inventory(InventoryEntry *he);
 extern int prepare_inventory(InventoryEntry *he);
+extern int add_post_prepare_cb(int (*actor)(void *data), void *data);
 
 #define LAST_PID_PATH		"sys/kernel/ns_last_pid"