meson: enable -Wgnu-variable-sized-type-not-at-end again

Follow-up for ab29e77aa9.

For gcc, flex-array-member-not-at-end was enabled by the commit,
but the option for clang with the same effect was still disabled.
Let's reenable it.

Prompted by #37497.

meson.build

@@ -489,7 +489,6 @@ endif
 if cc.get_id() == 'clang'
         possible_common_cc_flags += [
                 '-Wno-typedef-redefinition',
-                '-Wno-gnu-variable-sized-type-not-at-end',
         ]
 endif