Ignore autogenerated .prereqs.

Change-Id: I593d6bc7776d73e8dff6b9791242fdc3b18db338
diff --git a/.gitignore b/.gitignore
index f9e2644..798b081 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,4 @@
 *.pem
 accrc
 .stackenv
+.prereqs