pgsql: Fix "variable set but not used" warning with unnamed POSIX semap

Fix "variable set but not used" warning with unnamed POSIX semaphores.

Late-model clang is able to notice, when building posix_sema.c with
USE_NAMED_POSIX_SEMAPHORES not defined, that the static variable
nextSemKey is set once and read nowhere. Rearrange the #ifdef's
to not declare or set it unless USE_NAMED_POSIX_SEMAPHORES. As a
small side benefit, we can skip a stat() call altogether if we're
not going to need it to initialize this variable, which I think
clarifies the code a fraction.

Per buildfarm member fritillary.

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e406c4e7ad22f0140d73a19eaa0765e5d765283e

Modified Files
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src/backend/port/posix_sema.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)