Fix //third_party/sqlite BUILD.gn on iOS.
Fix //third_party/sqlite BUILD.gn on iOS.

The variable "defines" does not exists at that point, so it is not
possible to add to it. Instead just set it.

BUG=459705

Review URL: https://codereview.chromium.org/1255463002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#340656}

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