disable darwin build for libfstab

It broke mac sdk build

Change-Id: I5435cea7747d36c2ea46ed2e6c1c51bb032c6585
diff --git a/fs_mgr/Android.bp b/fs_mgr/Android.bp
index bf58315..d9f2837 100644
--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -99,6 +99,11 @@
         "fs_mgr_boot_config.cpp",
         "fs_mgr_slotselect.cpp",
     ],
+    target: {
+        darwin: {
+            enabled: false,
+        },
+    },
     export_include_dirs: ["include_fstab"],
     header_libs: ["libbase_headers"],
 }