pgsql: Fix update-po for the PGXS case

Fix update-po for the PGXS case

The original formulation failed to take into account the fact that for
the PGXS case, the source dir is not $(top_srcdir), so it ended up not
doing anything.  Handle it explicitly.

Author: Ryo Matsumura <matsumura.ryo@fujitsu.com>
Reviewed-by: Bryan Green <dbryan.green@gmail.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/TYCPR01MB113164770FB0B0BE6ED21E68EE8DCA@TYCPR01MB11316.jpnprd01.prod.outlook.com

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6aa04a60cbf5d7460c5444822b3127f5bdbc7930

Modified Files
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src/nls-global.mk | 5 +++++
1 file changed, 5 insertions(+)