Explicitly disable cpp-define-generator for Darwin.

This is (one of the many things) currently breaking the Mac build.

Test: builds on my puny Mac
Change-Id: Ifab20b357573917fb4929cf41052b2ed544476fd
diff --git a/tools/cpp-define-generator/Android.bp b/tools/cpp-define-generator/Android.bp
index 1f0d264..ca0c7a6 100644
--- a/tools/cpp-define-generator/Android.bp
+++ b/tools/cpp-define-generator/Android.bp
@@ -42,6 +42,11 @@
     out: ["asm_defines.h"],
     tool_files: ["make_header.py"],
     cmd: "$(location make_header.py) \"$(in)\" > \"$(out)\"",
+    target: {
+        darwin: {
+            enabled: false,
+        },
+    },
 }
 
 cc_library_headers {