vulkan: fix use of assert() with older meson versions

Follow-up to !1307

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1308>

ext/vulkan/meson.build

@@ -10,7 +10,8 @@ if not gstvulkan_dep.found()
   endif
 endif
 
-assert(glslc.found())
+# Should already have checked for this
+assert(glslc.found(), 'Expected glslc to be available')
 
 subdir('shaders')