vulkaninfo: Use app's extension count, not system

Was telling the driver to enable more extensions than
intended.
diff --git a/demos/vulkaninfo.c b/demos/vulkaninfo.c
index 2e7bfbd..7ca85d9 100644
--- a/demos/vulkaninfo.c
+++ b/demos/vulkaninfo.c
@@ -415,7 +415,7 @@
     info.queueRecordCount = gpu->queue_count;
     info.pRequestedQueues = gpu->queue_reqs;
 
-    info.extensionCount = extCount;
+    info.extensionCount = gpu->device_extension_count;
     info.pEnabledExtensions = enable_extension_list;
     dev->gpu = gpu;
     err = vkCreateDevice(gpu->obj, &info, &dev->obj);