Minor header cleanup for the new iovec code.

Remove redundant function declaration and improve header comment in
pg_iovec.h.  Move the new declaration in fd.h next to a group of more
similar functions.

src/include/port/pg_iovec.h

@@ -1,7 +1,7 @@
 /*-------------------------------------------------------------------------
  *
  * pg_iovec.h
- *	  Header for the vectored I/O functions in src/port/p{read,write}.c.
+ *	  Header for vectored I/O functions, to use in place of <sys/uio.h>.
  *
  * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -51,9 +51,4 @@ extern ssize_t pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offs
 extern ssize_t pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);
 #endif
 
-extern ssize_t pg_pwritev_with_retry(int fd,
-									 const struct iovec *iov,
-									 int iovcnt,
-									 off_t offset);
-
 #endif							/* PG_IOVEC_H */

src/include/storage/fd.h

@@ -154,6 +154,10 @@ extern int	pg_fsync_no_writethrough(int fd);
 extern int	pg_fsync_writethrough(int fd);
 extern int	pg_fdatasync(int fd);
 extern void pg_flush_data(int fd, off_t offset, off_t amount);
+extern ssize_t pg_pwritev_with_retry(int fd,
+									 const struct iovec *iov,
+									 int iovcnt,
+									 off_t offset);
 extern int	pg_truncate(const char *path, off_t length);
 extern void fsync_fname(const char *fname, bool isdir);
 extern int	fsync_fname_ext(const char *fname, bool isdir, bool ignore_perm, int elevel);
@@ -162,10 +166,6 @@ extern int	durable_unlink(const char *fname, int loglevel);
 extern int	durable_rename_excl(const char *oldfile, const char *newfile, int loglevel);
 extern void SyncDataDirectory(void);
 extern int	data_sync_elevel(int elevel);
-extern ssize_t pg_pwritev_with_retry(int fd,
-									 const struct iovec *iov,
-									 int iovcnt,
-									 off_t offset);
 
 /* Filename components */
 #define PG_TEMP_FILES_DIR "pgsql_tmp"