Fix a path to dx on Mac.

Change-Id: I7c5efdb75cb53bebc2b24760ce6e8303acefc59a
diff --git a/build.gradle b/build.gradle
index c734ad5..fdcc3bc 100644
--- a/build.gradle
+++ b/build.gradle
@@ -596,6 +596,8 @@
             dependsOn: ["jar_debuginfo_examples", "downloadDeps"]) {
         if (OperatingSystem.current().isWindows()) {
             executable file("tools/windows/dx/bin/dx.bat")
+        } else if (OperatingSystem.current().isMacOsX()) {
+            executable file("tools/mac/dx/bin/dx");
         } else {
             executable file("tools/linux/dx/bin/dx");
         }