Add musl benchmarks.

Test: Unit tests.
Change-Id: Ifb2911825f84b95fe64a803bfabd32fb81210eae
diff --git a/benchmarks/Android.bp b/benchmarks/Android.bp
index fae6f20..93f62cf 100644
--- a/benchmarks/Android.bp
+++ b/benchmarks/Android.bp
@@ -35,6 +35,7 @@
         "string_benchmark.cpp",
         "time_benchmark.cpp",
         "unistd_benchmark.cpp",
+        "stdlib_benchmark.cpp",
     ],
     shared_libs: ["libtinyxml2"],
     static_libs: [