pgsql: postgres_fdw: Mark statistics import helpers as static

postgres_fdw: Mark statistics import helpers as static

The set_*_arg helper functions in postgres_fdw.c are declared
static, but their definitions omitted the static keyword. Add it to
make their file-local scope explicit and keep the declarations and
definitions consistent.

Also fix a couple of nearby comment typos.

This is a followup to commit 54cd6fc8317.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwGjcQ4SwHMUQ9P8UYQ7iLKL1QE3uLSdONToQ1MrzpUUoQ@mail.gmail.com
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2bbec7c49a6c872fdfdddf7ff9ea9dc9cebaef40

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 12 ++++++------
src/backend/statistics/attribute_stats.c | 2 +-
src/backend/statistics/relation_stats.c | 2 +-
3 files changed, 8 insertions(+), 8 deletions(-)