pgsql: Include for older macOS.

Include <xlocale.h> for older macOS.

Commit 35eeea62 forgot to include <xlocale.h> when using locale_t
(which didn't seem to be required on newer Apple SDK as used by CI,
hence mistake).  Let's see if this fixes build farm animals longfin and
sifika.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/52ea7f0e0545e8b7e5f28cceddadbfb184f41a4f

Modified Files
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src/port/chklocale.c | 4 ++++
1 file changed, 4 insertions(+)