compel: Drop common/page.h from UAPI headers

This one is needed only for task_size() on some arches and it is
simpler to keep this routine in compel .c rather than messing
with common/page.h installation.

https://travis-ci.org/xemul/criu/builds/177585567


Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>