Build skottie_tool on Linux only

MoltenVK builds are failing on Mac.  Let's limit skottie_tool to Linux
for now, as that's the only platform Fiddle runs on.

Change-Id: Icc94e0e380d6f1bc32888dcaaeb812dad082f625
Reviewed-on: https://skia-review.googlesource.com/132404
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Hal Canary <halcanary@google.com>
1 file changed