MiraclePtr: Inject PA dependency in skia_target

This injects the src/partition_alloc:raw_ptr dependency into the
skia_target template in gn/skia.gni.

This ensures that standalone Skia builds (and other non-Chromium
clients using skia_target) get the partition_alloc dependency when
we start using raw_ptr in core headers.

For Chromium builds, this change has no effect as Chromium uses its
own skia_source_set template and does not load gn/skia.gni in a way
that causes clashes.

Bug: 351867706
Doc: https://bit.ly/miracleptr-skia
Change-Id: I79e815981c0e91b4c2d4a92c23d291762ac7a92d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1305416
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>