Fix link error for test_radixtree module on Windows
Add PGDLLIMPORT to pg_popcount32/64. In passing, fix a typo.
Diagnosis by Masahiko Sawada, patch by David Rowley
Per buildfarm members drongo and fairywren
Discussion:
https://postgr.es/m/CAD21AoAMm1mQd%3Dw4PrfrKK%3DOMP8j8%3D7ntJRPF8%2B%3D10i
Fix link error for test_radixtree module on Windows
Add back "link_with" directive, similar to the one removed by 1f1d73a8b,
but only for Windows, but use the "_shlib" variation.
Diagnosis by Masahiko Sawada, proposed fix adjusted and tested by me
Per buildfarm members drongo and fairywren
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