Make xlog_internal.h includable in frontend context.

Heikki Linnakangas <heikki.linnakangas@iki.fi>

Commit: 527668717a660e67c2a6cfd4e85f7a513f99f6f2
Author: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: 2012-12-13T12:59:13Z
Releases: 9.3.0
Make xlog_internal.h includable in frontend context.

This makes unnecessary the ugly hack used to #include postgres.h in
pg_basebackup.

Based on Alvaro Herrera's patch

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