Make gen_partprune_steps static

There's no need to export this function, so don't.  Michaël didn't
actually write the patch, but we list him as first author because with a
trivial one like this, intellectual authorship is as important (if not
more) as bit shovelling.

Author: Michaël Paquier, Amit Langote
Discussion: https://postgr.es/m/c91299c4-199b-0f16-339b-a29d6d2a39ee@lab.ntt.co.jp

src/backend/partitioning/partprune.c

@@ -116,6 +116,8 @@ typedef struct PruneStepResult
 } PruneStepResult;
 
 
+static List *gen_partprune_steps(RelOptInfo *rel, List *clauses,
+					bool *contradictory);
 static List *gen_partprune_steps_internal(GeneratePruningStepsContext *context,
 							 RelOptInfo *rel, List *clauses,
 							 bool *contradictory);
@@ -355,7 +357,7 @@ make_partition_pruneinfo(PlannerInfo *root, List *partition_rels,
  * If the clauses in the input list are contradictory or there is a
  * pseudo-constant "false", *contradictory is set to true upon return.
  */
-List *
+static List *
 gen_partprune_steps(RelOptInfo *rel, List *clauses, bool *contradictory)
 {
 	GeneratePruningStepsContext context;

src/include/partitioning/partprune.h

@@ -67,7 +67,5 @@ extern List *make_partition_pruneinfo(PlannerInfo *root, List *partition_rels,
 extern Relids prune_append_rel_partitions(RelOptInfo *rel);
 extern Bitmapset *get_matching_partitions(PartitionPruneContext *context,
 						List *pruning_steps);
-extern List *gen_partprune_steps(RelOptInfo *rel, List *clauses,
-					bool *contradictory);
 
 #endif							/* PARTPRUNE_H */