contrib/plugins: remove -soname argument

contrib/plugins: remove -soname argument

-soname is not needed for runtime-loaded modules.  For example, Meson says:

            if not isinstance(target, build.SharedModule) or target.force_soname:
                # Add -Wl,-soname arguments on Linux, -install_name on OS X
                commands += linker.get_soname_args(
                    self.environment, target.prefix, target.name, target.suffix,
                    target.soversion, target.darwin_versions)

(force_soname is set is shared modules are linked into a build target, which is not
the case here.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>