MODSIGN: fix a compilation warning in extract-cert

Fix the following warning when compiling extract-cert:

  scripts/extract-cert.c: In function `write_cert':
  scripts/extract-cert.c:89:2: warning: format not a string literal and no format arguments [-Wformat-security]
    ERR(!i2d_X509_bio(wb, x509), cert_dst);
    ^

whereby the ERR() macro is taking cert_dst as the format string.  "%s"
should be used as the format string as the path could contain special
characters.
Signed-off-by: David Howells <dhowells@redhat.com>
Reported-by: Jim Davis <jim.epost@gmail.com>
Acked-by : David Woodhouse <david.woodhouse@intel.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/extract-cert.c

@@ -86,7 +86,7 @@ static void write_cert(X509 *x509)
 		ERR(!wb, "%s", cert_dst);
 	}
 	X509_NAME_oneline(X509_get_subject_name(x509), buf, sizeof(buf));
-	ERR(!i2d_X509_bio(wb, x509), cert_dst);
+	ERR(!i2d_X509_bio(wb, x509), "%s", cert_dst);
 	if (kbuild_verbose)
 		fprintf(stderr, "Extracted cert: %s\n", buf);
 }