[skottie] Enable skotte_tool on Mac builds

TBR=
Change-Id: I6896c6d52c7f85e5c967920340f873b3f1daf4b3
Reviewed-on: https://skia-review.googlesource.com/151160
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 4350eb6..e03fe06 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1539,7 +1539,7 @@
     ]
   }
 
-  if (is_linux) {
+  if (is_linux || is_mac) {
     test_app("skottie_tool") {
       deps = [
         "modules/skottie:tool",