[gtk+] GdkDrawingContext: Add missing nullable annotation

[Timm Bäder <baedert src gnome org>]

commit b6ee3490c3336e8a2ff700a2787cc456787a5247
Author: Timm Bäder <mail baedert org>
Date:   Mon Jan 8 22:00:57 2018 +0100

    GdkDrawingContext: Add missing nullable annotation

 gdk/gdkdrawingcontext.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gdk/gdkdrawingcontext.c b/gdk/gdkdrawingcontext.c
index 31ab477..d0ae2a4 100644
--- a/gdk/gdkdrawingcontext.c
+++ b/gdk/gdkdrawingcontext.c
@@ -260,7 +260,7 @@ gdk_cairo_get_drawing_context (cairo_t *cr)
  * #GdkDrawingContext is valid, that is between a call to
  * gdk_window_begin_draw_frame() and gdk_window_end_draw_frame().
  *
- * Returns: (transfer none): a Cairo context to be used to draw
+ * Returns: (transfer none) (nullable): a Cairo context to be used to draw
  *   the contents of the #GdkWindow. The context is owned by the
  *   #GdkDrawingContext and should not be destroyed. %NULL is
  *   returned when a paint context is in used.