Include bionic/Android.bp

bionic/Android.bp exists now, and subdirectories are handled with
Android.soong.mk files.

Change-Id: I70259ff5c2fb9df2f8716590502f49de1c2c8ac7
diff --git a/root.bp b/root.bp
index 8ba1c12..2b72cfa 100644
--- a/root.bp
+++ b/root.bp
@@ -6,7 +6,7 @@
 ]
 
 optional_subdirs = [
-    "bionic/*",
+    "bionic",
     "build/kati",
     "build/tools/*",
     "dalvik",