layers: Add S8_UINT to FormatIsUINT() helper

Fixed an oversight in FormatIsUINT() helper function, add VK_FORMAT_S8_UINT
to the list.

Change-Id: Ie01c2dbd1db7cdae1a4939543c00aa7b32ae31ff
diff --git a/layers/vk_format_utils.cpp b/layers/vk_format_utils.cpp
index 7d9c084..ce5895e 100644
--- a/layers/vk_format_utils.cpp
+++ b/layers/vk_format_utils.cpp
@@ -542,6 +542,7 @@
 
     switch (format) {
         case VK_FORMAT_R8_UINT:
+        case VK_FORMAT_S8_UINT:
         case VK_FORMAT_R8G8_UINT:
         case VK_FORMAT_R8G8B8_UINT:
         case VK_FORMAT_R8G8B8A8_UINT: