Fix missing dependency for the OpenGL loader generator.

This was causing the GL loader to not be rebuilt when
supported_extensions.json was written.

Bug: None
Change-Id: I1f085403ebabe3a4cf52a80bcc7d23217603c28b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/49888
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

generator/opengl_loader_generator.py

@@ -273,7 +273,10 @@ class OpenGLLoaderGenerator(Generator):
         ]
 
     def get_dependencies(self, args):
-        return [os.path.abspath(args.gl_xml)]
+        return [
+            os.path.abspath(args.gl_xml),
+            os.path.abspath(args.supported_extensions)
+        ]
 
 
 if __name__ == '__main__':