Include the dep itself in extra_paths for Windows

This is needed in the case where a custom_target directly depends on a
shared library, and somehow loads it.

(Specifically this can be the case with gtkdoc, when it invokes
gtkdoc-scangobj, which will build and run it's own code to load a shared
library, to introspect it)

mesonbuild/backend/backends.py

@@ -743,6 +743,7 @@ class Backend:
             for deppath in self.rpaths_for_bundled_shared_libraries(target, exclude_system=False):
                 result.add(os.path.normpath(os.path.join(self.environment.get_build_dir(), deppath)))
         for bdep in extra_bdeps:
+            prospectives.add(bdep)
             prospectives.update(bdep.get_transitive_link_deps())
         # Internal deps
         for ld in prospectives: