pstree.h: mark dmpi's arg as const

We'll need this for the next bits and the series, and since dmpi doesn't
modify this or even use it the structure, it's safe.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/pstree.h

@@ -47,7 +47,7 @@ struct dmp_info {
 	struct parasite_ctl *parasite_ctl;
 };
 
-static inline struct dmp_info *dmpi(struct pstree_item *i)
+static inline struct dmp_info *dmpi(const struct pstree_item *i)
 {
 	return (struct dmp_info *)(i + 1);
 }