Adding 'postinstall' root dir unconditionally.

It is required to pass update_engine_unittests in GSI
compliance test. And it's clean to just add this mount
dir unconditionally.

Bug: 172696594
Test: `m init.environ.rc` and checks that $OUT/root/postinstall exists
Change-Id: Ib340a78af442ea66c45cecb373a9eb3c428f8dda
Merged-In: Ib340a78af442ea66c45cecb373a9eb3c428f8dda
1 file changed