Move tar function headers to pgtar.h

This makes it possible to include them only where they are used, so
we can avoid the conflict of the uid_t and gid_t datatypes that happened
in plperl (since plperl doesn't need the tar functions)

src/backend/replication/basebackup.c

@@ -32,6 +32,7 @@
 #include "utils/builtins.h"
 #include "utils/elog.h"
 #include "utils/ps_status.h"
+#include "pgtar.h"
 
 typedef struct
 {