Create build/generated dir if needed

Change-Id: I6a72e7d78333a18a1789579e19c5e0c3a5b55bce
diff --git a/build.gradle b/build.gradle
index b4b63dd..67d455a 100644
--- a/build.gradle
+++ b/build.gradle
@@ -832,6 +832,7 @@
         }
     }
     doLast {
+        file("build/generated").mkdir()
         def file = file("build/generated/supportlibraries.txt")
         file.createNewFile()
         file.text = ""