Do not build any host targets on mac.

Bug: 23762183

Test: Compiled.
Change-Id: I42afabeb7d3576ea4afd4af05bb1cabd79ac76e5
diff --git a/libunwindstack/Android.bp b/libunwindstack/Android.bp
index 63768b2..9bb1304 100644
--- a/libunwindstack/Android.bp
+++ b/libunwindstack/Android.bp
@@ -24,6 +24,12 @@
         "-Werror",
         "-Wextra",
     ],
+
+    target: {
+        darwin: {
+            enabled: false,
+        },
+    },
 }
 
 cc_defaults {
@@ -97,9 +103,6 @@
     },
 
     target: {
-        darwin: {
-            enabled: false,
-        },
         linux: {
             host_ldlibs: [
                 "-lrt",