Add direct include to pairingheap.h in headers

ivfflat.h and hnsw.h have references to pairingheap_node, so they need
to include lib/pairingheap.h. It happened to work, because
lib/pairingheap.h was being included indirectly through
nodes/execnodes.h, but let's be explicit.

Remove the include from hnswbuild.c, because there are no calls to
pairingheap functions in that file. Instead, add the includes to
hnswutils.c and ivfscan.c, which do have such calls. They are not
strictly necessary again because of the indirect include from hnsw.h
and ivfflat.h, but let's be explicit while we're messing with this.

src/hnsw.h

@@ -6,6 +6,7 @@
 #include "access/generic_xlog.h"
 #include "access/parallel.h"
 #include "access/reloptions.h"
+#include "lib/pairingheap.h"
 #include "nodes/execnodes.h"
 #include "port.h"				/* for random() */
 #include "utils/relptr.h"

src/hnswbuild.c

@@ -46,7 +46,6 @@
 #include "commands/progress.h"
 #include "hnsw.h"
 #include "miscadmin.h"
-#include "lib/pairingheap.h"
 #include "optimizer/optimizer.h"
 #include "storage/bufmgr.h"
 #include "tcop/tcopprot.h"

src/hnswutils.c

@@ -3,6 +3,7 @@
 #include <math.h>
 
 #include "hnsw.h"
+#include "lib/pairingheap.h"
 #include "storage/bufmgr.h"
 #include "utils/datum.h"
 #include "vector.h"

src/ivfflat.h

@@ -6,6 +6,7 @@
 #include "access/generic_xlog.h"
 #include "access/parallel.h"
 #include "access/reloptions.h"
+#include "lib/pairingheap.h"
 #include "nodes/execnodes.h"
 #include "port.h"				/* for random() */
 #include "utils/sampling.h"

src/ivfscan.c

@@ -5,6 +5,7 @@
 #include "access/relscan.h"
 #include "catalog/pg_operator_d.h"
 #include "catalog/pg_type_d.h"
+#include "lib/pairingheap.h"
 #include "ivfflat.h"
 #include "miscadmin.h"
 #include "pgstat.h"