Use -Werror in external/selinux

* Use -Wno-error= to keep existing warnings to fix later.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: Id7d641c73f396c37c1fb8c5228919c722e2bedd3
(cherry picked from commit f48068e8dc466ef2d8f01cda98c0ed4efcc9055e)
3 files changed