libotautil exports libfstab header.

otautil/roots.h includes <fstab/fstab.h>, but users of otautil/roots.h
don't need to explicitly depend on libfstab unless they have a real
need.

Also remove the unneeded include of <fstab/fstab.h> from
fsck_unshare_blocks.cpp.

Test: mmma -j bootable/recovery
Change-Id: Id3dc995a4769e631ab242843ee439bd94b2bf0bc