Blind attempt at fixing isolation_tester on Win32

src/tools/msvc/Mkvcbuild.pm

@@ -284,6 +284,7 @@ sub mkvcbuild
     $isolation_tester->AddIncludeDir('src\interfaces\libpq');
     $isolation_tester->AddDefine('HOST_TUPLE="i686-pc-win32vc"');
     $isolation_tester->AddDefine('FRONTEND');
+    $isolation_tester->AddLibrary('wsock32.lib');
     $isolation_tester->AddReference($libpq, $libpgport);
 
     my $pgregress_isolation = $solution->AddProject('pg_isolation_regress','exe','misc');