[iOS] Defaults to use system libjpeg on iOS.
[iOS] Defaults to use system libjpeg on iOS.

It is currently not possible to build libjpeg_turbo on iOS (and may
not be a good idea in term of application size) so default to using
the version of libjpeg shipped with the system.

Move the "libs" assignment from the "jpeg" to "system_libjpeg_config"
config.

BUG=459705

Review URL: https://codereview.chromium.org/1806503002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381432}

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