Add phpcs exclusion for strict_types declararion

Summary: We are not yet compliant and newer versions of phpcs give linter failures, so we should exclude this for now.

Test Plan:

phpcs --version  # I'm running 3.5.0, which is newer and errors on strict_types
phpcs <any linter in /arcanist/linter/>  # Without patch: `1 | ERROR | Missing required strict_types declaration`
phpcs <any linter in /arcanist/linter/>  # With patch: (no error)

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4193