pgsql: Add missing include in Cluster.pm

Add missing include in Cluster.pm

The postmaster test 004_negotiate.pl could fail due to IO::Socket::INET
gone missing, in environments that cannot use Unix sockets.

Oversight in the backport done in 6dffaeb8e54c, so like the other commit
this is applied across the v14~17 range. Per buildfarm member drongo.

Security: CVE-2026-6479
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d8d46710cdbb0b0cfb9a8d614b7e8937297aff58

Modified Files
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src/test/perl/PostgreSQL/Test/Cluster.pm | 1 +
1 file changed, 1 insertion(+)