fiddle_build_test/gyp: fix mac mesa build

TBR=
NOTRY=true

Review URL: https://codereview.chromium.org/1900143002
diff --git a/gyp/tools.gyp b/gyp/tools.gyp
index 9c5cfab..6c96ca5 100644
--- a/gyp/tools.gyp
+++ b/gyp/tools.gyp
@@ -591,7 +591,7 @@
         ],
       },
     ],
-    ['skia_os == "linux"',
+    ['skia_os in ["linux", "mac"]',
       {
         'targets': [
           {