Increase statistics targets for the derivation_inputs table

It has a lot of rows, and this seems to help the query planning for
select-required-builds-that-failed, as I think it helps PostgreSQL better
estimate the row counts that different parts of the query will produce.

sqitch/verify/increase_derivation_inputs_statistics_targets.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:increase_derivation_inputs_statistics_targets on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;