GutterRenderer: fix coding style

renderer->priv->view is a GtkTextView, so we check if it's a
GtkSourceView. Once we know we have a GtkSourceView, we can assume that
the buffer is a GtkSourceBuffer and that the style scheme is NULL or
has the right type.

Also, add curly braces around one-line blocks.
GutterRendererText: fix compilation warnings

About unused variables.