fix: force linking of shared libraries during build

[gf3]

Force linking of shared libraries (libpostal) at build time instead of at runtime.

This fixes the issue of the built library (expostal.so) being unable to lookup symbols from libpostal at runtime on some systems.

expostal.so: undefined symbol: libpostal_expand_address

[gf3]

For others who might find this, I have also opened a corresponding PR on the as-of-now most up-to-date fork → https://github.com/dkuku/expostal/pull/1