Add missing inline keyword for constants in language_pack_util.h

Follow guidance on https://groups.google.com/a/chromium.org/g/chromium-dev/c/OXxJy5L96jI/m/tRZHIeZwAgAJ
and add the missing inline keyword so they only have one definition.
Also do IWYU in this header.

Change-Id: I485e3fc00c55b1008d7d400d71ae1c90612001f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6527133
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457920}