vkEnumerateLayers: Update to match upstream

The upstream vulkan.h header removed maxLayerCount parameter
from vkEnumerateLayers. This patch implements that change
for the sample driver and layers.
diff --git a/vulkan.py b/vulkan.py
index d9a0775..e77a662 100755
--- a/vulkan.py
+++ b/vulkan.py
@@ -262,9 +262,8 @@
 
         Proto("VkResult", "EnumerateLayers",
             [Param("VkPhysicalDevice", "gpu"),
-             Param("size_t", "maxLayerCount"),
              Param("size_t", "maxStringSize"),
-             Param("size_t*", "pOutLayerCount"),
+             Param("size_t*", "pLayerCount"),
              Param("char* const*", "pOutLayers"),
              Param("void*", "pReserved")]),