cr-restore: move add_ns_shared_cb declaration to include/namespaces.h

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/namespaces.h

@@ -134,4 +134,6 @@ extern int __userns_call(const char *func_name, uns_call_t call, int flags,
 	__userns_call(__stringify(__call), __call, __flags,	\
 		      __arg, __arg_size, __fd)
 
+extern int add_ns_shared_cb(int (*actor)(void *data), void *data);
+
 #endif /* __CR_NS_H__ */