pstree.h: mark dmpi's arg as const
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 <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu