pgsql: Remove useless extern keywords

Remove useless extern keywords

An extern keyword on a function definition (not declaration) is
useless and not the normal style.

Discussion: /messages/by-id/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3fb59e789dd9f21610101d1ec106ad58095e24f3

Modified Files
--------------
src/backend/backup/basebackup_incremental.c | 4 ++--
src/backend/storage/file/fd.c | 2 +-
src/bin/pg_basebackup/bbstreamer_inject.c | 2 +-
src/bin/pg_basebackup/bbstreamer_tar.c | 4 ++--
4 files changed, 6 insertions(+), 6 deletions(-)