[Chromecast] Add chromecast/public to fuchsia test include path

chromecast/public uses relative paths for include, because OEM may have
different directory structures.

Fuchsia backend unit tests should add chromecast/public to include path.
This is to fix the build failure introduced by CL:
https://chromium-review.googlesource.com/c/chromium/src/+/898437

BUG=internal b/64914604
TEST=Build fuschia unit tests.

Change-Id: I10081c7c7a566307ea4d4019336f2de23cd719f8
Reviewed-on: https://chromium-review.googlesource.com/912243
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536208}
1 file changed