vk_validation_stats: Support nested build

In nested builds, cmake output is in the toplevel of the root.
Look in here as well.
diff --git a/scripts/vk_validation_stats.py b/scripts/vk_validation_stats.py
index 7fbf0e4..7c72653 100755
--- a/scripts/vk_validation_stats.py
+++ b/scripts/vk_validation_stats.py
@@ -52,6 +52,7 @@
 'build',
 'dbuild',
 'release',
+'../build/Vulkan-ValidationLayers/'
 ]
 generated_layer_source_files = [
 'parameter_validation.cpp',