layers: Add missing promoted device extensions

Add three missing device extensions to promoted device extensions list
in vk_extension_helper codegen.

    VK_KHR_relaxed_block_layout
    VK_KHR_shader_draw_parameters
    VK_KHR_storage_buffer_storage_class

Change-Id: I392155d318ca580e576436ef46695218e5eb446e
diff --git a/scripts/helper_file_generator.py b/scripts/helper_file_generator.py
index dd82bbb..c1b5176 100644
--- a/scripts/helper_file_generator.py
+++ b/scripts/helper_file_generator.py
@@ -492,9 +492,13 @@
             'vk_khr_maintenance2',
             'vk_khr_maintenance3',
             'vk_khr_multiview',
+            'vk_khr_relaxed_block_layout',
             'vk_khr_sampler_ycbcr_conversion',
+            'vk_khr_shader_draw_parameters',
+            'vk_khr_storage_buffer_storage_class',
             'vk_khr_variable_pointers',
             ]
+
         output = [
             '',
             '#ifndef VK_EXTENSION_HELPER_H_',