otautil: roots.cpp no longer needs 'mounts.h'.

Commit 0f339e27 moved part of the mounts
implementation into libfs_mgr. As a result, otautil/roots.cpp no longer
depends on anything in the local otautil/mounts.h.

Test: mmma bootable/recovery
Change-Id: If16c3e19a62933358fb0002a10e8556a99f9d29a