[specfile] fix postinstall and preuninstall scripts

Summary:
If watchman wasn't already running, the postinstall and preuninstall scripts
would fail.

Test Plan: No obvious way to test this.

Reviewers: wez

Differential Revision: https://phabricator.fb.com/D1009042

Task ID: 2825133

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extra : amend_source : 1a94bd3f150c728eed524b23c5ab2679ab6ff4c8