Add android compat headers

Add sys/cdefs.h and unistd.h that provide __BEGIN_DECLS/__END_DECLS
and TEMP_FAILURE_RETRY.

Bug: 190084016
Test: build with musl
Change-Id: I145734ada32632cc209c65e10819c8d3504dc6e6
diff --git a/Android.bp b/Android.bp
index 7570a32..1ecf6c6 100644
--- a/Android.bp
+++ b/Android.bp
@@ -72,6 +72,7 @@
     device_supported: false,
     system_shared_libs: [],
     export_system_include_dirs: [
+        "android/include",
         "include",
     ],
 }