gnome.gtkdoc: Fix static library ldflags

When passing static libraries to gtkdoc, they are also appended as
shared libraries to ldflags, which makes the process to fail.

This has been changed to only append shared libraries to ldflags.

Fixes #3935

mesonbuild/modules/gnome.py

@@ -331,6 +331,7 @@ class GnomeModule(ExtensionModule):
                 for lib in dep.libraries:
                     if hasattr(lib, 'held_object'):
                         lib = lib.held_object
+                    if isinstance(lib, build.SharedLibrary):
                         internal_ldflags.update(self._get_link_args(state, lib, depends, include_rpath))
                         libdepflags = self._get_dependencies_flags(lib.get_external_deps(), state, depends, include_rpath,
                                                                    use_gir_args, True)