Add get_dependencies method to Swift-DocC-Render script

Adding the required `get_dependencies` class method should resolve the remaining test failures we're seeing for macOS toolchain builds.

Author: ethan-kusters

utils/swift_build_support/swift_build_support/products/swiftdoccrender.py

@@ -66,3 +66,7 @@ def install(self, host_target):
         shell.call(["mkdir", "-p", install_path])
         shell.call(
             ["rsync", "-a", artifact_dist_path_with_trailing_slash, install_path])
+
+    @classmethod
+    def get_dependencies(cls):
+        return []