-Wunused-but-set-global in scripts/mod/modpost.c

[nathanchance]

After llvm/llvm-project@fd11cf4 (llvm/llvm-project@fd11cf4 split the warning out from -Wunused-but-set-variable into -Wunused-but-set-global):

scripts/mod/modpost.c:59:13: error: variable 'extra_warn' set but not used [-Werror,-Wunused-but-set-global]
   59 | static bool extra_warn;
      |             ^

I submitted https://lore.kernel.org/20260325-modpost-extra_warn-unused-but-set-global-v1-1-2e84003b7e81@kernel.org/.