vk/util: fix 'beta' check for physical device properties

`--beta` is a string, not a bool (although really it should be, but that's a bigger change).

Fixes: 083793a39d ("vulkan: Allow beta extensions for physical device properties")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27394>
(cherry picked from commit c35247ab20)

src/vulkan/util/vk_physical_device_properties_gen.py

@@ -223,7 +223,7 @@ def get_property_structs(doc, api, beta):
 
         # Skip extensions with a define for now
         guard = required[full_name].guard
-        if guard is not None and (guard != "VK_ENABLE_BETA_EXTENSIONS" or not beta):
+        if guard is not None and (guard != "VK_ENABLE_BETA_EXTENSIONS" or beta != "true"):
             continue
 
         # find Vulkan structure type