libcutils: create private/fs_config.h

(cherry picked from commit a9403f0d)

private/fs_config.h is required in order to build an independent
test that requires internal binary knowledge of the
etc/fs_config_(files|dirs) files.

Test: compile
Bug: 36071012
Change-Id: I268bcfdbb6d45b7bf6040cbf307a4e34812f5fef