build: avoid warnings in daemon/inotify.c

Avoid "comparison between signed and unsigned integer expressions"
warnings.  If it's at all hard or risky to avoid this type of warning,
then it's not worthwhile.  Here, it's easy and safe.
* daemon/inotify.c (inotify_posn): Declare local to be of unsigned type.
(do_inotify_read, do_inotify_files): Likewise.