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 6dffaeb8e5, so like the other commit
this is applied across the v14~17 range.  Per buildfarm member drongo.

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

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -107,6 +107,7 @@ use File::Path qw(rmtree mkpath);
 use File::Spec;
 use File::stat qw(stat);
 use File::Temp ();
+use IO::Socket::INET;
 use IPC::Run;
 use PostgreSQL::Version;
 use PostgreSQL::Test::RecursiveCopy;