libbacktrace: add benchmarks for Backtrace::Create, CreateNew.

Test: backtrace_benchmarks
Change-Id: I80ba5adb31d446314060edd00149f09e14e6815e
diff --git a/libbacktrace/Android.bp b/libbacktrace/Android.bp
index b2c0c0f..ed10183 100644
--- a/libbacktrace/Android.bp
+++ b/libbacktrace/Android.bp
@@ -230,5 +230,6 @@
     shared_libs: [
         "libbacktrace",
         "libbase",
+        "libunwindstack",
     ],
 }