pgsql: Silence -Wmissing-variable-declarations in headerscheck.

Silence -Wmissing-variable-declarations in headerscheck.

Newer gcc versions will emit warnings about missing extern
declarations if certain header files are compiled by themselves.
Add the "extern" declarations needed to quiet that.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/1127775.1754417387@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8722e7965fd24bf94c278fb86cab99d0c5360532

Modified Files
--------------
src/interfaces/ecpg/test/expected/preproc-strings.c | 15 ++++++++++++++-
src/interfaces/ecpg/test/preproc/strings.h          | 10 ++++++++++
src/tools/gen_keywordlist.pl                        | 11 ++++++++++-
3 files changed, 34 insertions(+), 2 deletions(-)