-Wunused-but-set-global in certs/extract-cert.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):

certs/extract-cert.c:46:20: error: variable 'key_pass' set but not used [-Werror,-Wunused-but-set-global]
   46 | static const char *key_pass;
      |                    ^

I submitted https://lore.kernel.org/20260325-certs-extract-cert-key_pass-unused-but-set-global-v1-1-ecf94326d532@kernel.org/.