add GenericPretty to extra-deps to satisfy lts-9.21

Author: mrd

stack.yaml

@@ -8,7 +8,8 @@ resolver: lts-9.21
 packages:
 - '.'
 # Packages to be pulled from upstream that are not in the resolver (e.g., acme-missiles-0.3)
-extra-deps: []
+extra-deps:
+- GenericPretty-1.2.1
 
 # Override default flag values for local packages and extra-deps
 flags: {}