build: remove --export-dynamic

AFAICT --export-dynamic is only suitable when linking into executables. Since the usage here is for libraries, it seems kind of pointless to use it. Also it doesn't work on macOS (see #34 (closed)), where it is called -export_dynamic.

In fact meson actually offers the option export_dynamic for their executable instances, so if this were ever necessary for an executable, the option should be used instead of trying to add the link argument explicitly.