pgstat: move pgstat.c to utils/activity.

Now that pgstat is not related to postmaster anymore, src/backend/postmaster
is not a well fitting directory.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

src/backend/postmaster/Makefile

@@ -21,7 +21,6 @@ OBJS = \
 	fork_process.o \
 	interrupt.o \
 	pgarch.o \
-	pgstat.o \
 	postmaster.o \
 	shell_archive.o \
 	startup.o \

src/backend/utils/activity/Makefile

@@ -16,6 +16,7 @@ include $(top_builddir)/src/Makefile.global
 OBJS = \
 	backend_progress.o \
 	backend_status.o \
+	pgstat.o \
 	pgstat_archiver.o \
 	pgstat_bgwriter.o \
 	pgstat_checkpointer.o \

src/backend/utils/activity/pgstat.c

@@ -84,7 +84,7 @@
  * Copyright (c) 2001-2022, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  src/backend/postmaster/pgstat.c
+ *	  src/backend/utils/activity/pgstat.c
  * ----------
  */
 #include "postgres.h"