Fix duplicate typedef from commit 0d8c9c121.

Older gcc versions don't like duplicate typedefs, so get rid of
that in favor of doing it like we do it elsewhere, ie just use
a "struct" declaration when trying to avoid importing a whole
header file.

Also, there seems no reason to include stringinfo.h here at all,
so get rid of that addition too.

Discussion: https://postgr.es/m/27239.1587415696@sss.pgh.pa.us

src/include/replication/basebackup.h

@@ -12,16 +12,17 @@
 #ifndef _BASEBACKUP_H
 #define _BASEBACKUP_H
 
-#include "lib/stringinfo.h"
 #include "nodes/replnodes.h"
 
+struct manifest_info;			/* avoid including backup_manifest.h */
+
+
 /*
  * Minimum and maximum values of MAX_RATE option in BASE_BACKUP command.
  */
 #define MAX_RATE_LOWER	32
 #define MAX_RATE_UPPER	1048576
 
-
 typedef struct
 {
 	char	   *oid;
@@ -30,12 +31,9 @@ typedef struct
 	int64		size;
 } tablespaceinfo;
 
-struct manifest_info;
-typedef struct manifest_info manifest_info;
-
 extern void SendBaseBackup(BaseBackupCmd *cmd);
 
 extern int64 sendTablespace(char *path, char *oid, bool sizeonly,
-							manifest_info *manifest);
+							struct manifest_info *manifest);
 
 #endif							/* _BASEBACKUP_H */