Set libunwind's STL to none.

This avoids the unnecessary dependency on libstdc++.so.

Test: make checkbuild
Bug: None
Change-Id: I8c04e364b932d78494e2755a30d2a85bfcc11844
diff --git a/Android.bp b/Android.bp
index 538476a..f4653de 100644
--- a/Android.bp
+++ b/Android.bp
@@ -102,6 +102,7 @@
     name: "libunwind",
     defaults: ["libunwind_defaults"],
     sdk_version: "21",
+    stl: "none",
 
     srcs: [
         "src/mi/init.c",