Fix unused variable warning on Mac OS

Change-Id: I0808c3873685ed6563168b4ebd3c1d0bb7cf8ab2
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index ac428c6..5a9256e 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -449,8 +449,8 @@
         usage();
       }
     } else if (option.starts_with("-j")) {
-      const char* thread_count_str = option.substr(strlen("-j")).data();
 #if! defined(__APPLE__)
+      const char* thread_count_str = option.substr(strlen("-j")).data();
       // Apple lacks GetOwner and therefore working Mutexes, so we ignore -j on that platform
       if (!ParseInt(thread_count_str, &thread_count)) {
         fprintf(stderr, "could not parse -j argument '%s' as an integer\n", thread_count_str);