idmap: scan missing include for sys/stat.h

scan.cpp gets sys/stat.h inherited from
private/android_filesystem_config.h it should
not rely on this in the future. The intent is
to move fs_config function into libcutils and
thus deprecate any need for sys/stat.h in this
include file.

Bug: 19908228
Change-Id: If547e86513b06c536972138ae571c3d9c714ffe9