build: Add libffi as a build dependency on linux.
build: Add libffi as a build dependency on linux.

libffi is already a run-time dependency as libgobject depends on it.

BUG=549782

Review URL: https://codereview.chromium.org/1410093009

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#358662}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk