extract-cert: add static to local data

Fix the following warning from sparse:

  scripts/extract-cert.c:74:5: warning: symbol 'kbuild_verbose' was not declared. Should it be static?
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/extract-cert.c

@@ -71,7 +71,7 @@ static void drain_openssl_errors(void)
 static const char *key_pass;
 static BIO *wb;
 static char *cert_dst;
-int kbuild_verbose;
+static int kbuild_verbose;
 
 static void write_cert(X509 *x509)
 {