pgsql: Remove dependency to StringInfo in xlogbackup.{c.h}

Remove dependency to StringInfo in xlogbackup.{c.h}

This was used as the returned result type of the generated contents for
the backup_label and backup history files.  This is replaced by a simple
string, reducing the cleanup burden of all the callers of
build_backup_content().

Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/YzERvNPaZivHEKZJ@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e1e6f8f3dfe141839220875270a81ad3a719e0b6

Modified Files
--------------
src/backend/access/transam/xlog.c       |  5 ++---
src/backend/access/transam/xlogbackup.c | 10 +++++++---
src/backend/access/transam/xlogfuncs.c  |  5 ++---
src/backend/backup/basebackup.c         |  5 ++---
src/include/access/xlogbackup.h         |  5 ++---
5 files changed, 15 insertions(+), 15 deletions(-)