[gtk+] Quiet a compiler warning

[Matthias Clasen <matthiasc src gnome org>]

commit 888f289114756daeb80591afd48496fce1d63e8d
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Jan 9 17:20:44 2018 -0500

    Quiet a compiler warning
    
    Vulkan grew a new error code.

 gdk/gdkvulkancontext.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdkvulkancontext.c b/gdk/gdkvulkancontext.c
index cb14d3b..0e3d4f9 100644
--- a/gdk/gdkvulkancontext.c
+++ b/gdk/gdkvulkancontext.c
@@ -149,6 +149,7 @@ gdk_vulkan_strerror (VkResult result)
     case VK_ERROR_INVALID_EXTERNAL_HANDLE_KHR:
     case VK_RESULT_RANGE_SIZE:
     case VK_RESULT_MAX_ENUM:
+    case VK_ERROR_NOT_PERMITTED_EXT:
     default:
       return "Unknown Vulkan error.";
   }