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
diff --git a/otautil/Android.bp b/otautil/Android.bp
index b4936c0..c9ecba7 100644
--- a/otautil/Android.bp
+++ b/otautil/Android.bp
@@ -61,6 +61,10 @@
                 "libfs_mgr",
                 "libselinux",
             ],
+
+            export_static_lib_headers: [
+                "libfstab",
+            ],
         },
     },
 }