grub-mkconfig: Fix detecting .sig files as system images

grub-mkconfig detects detached RSA signatures for kernel images used for
signature checking as valid images and adds them to grub.cfg as separate
menu entries. This patch adds .sig extension to common blacklist.

Signed-off-by: Jordan Glover <Golden_Miller83@protonmail.ch>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub-mkconfig_lib.in

@@ -188,6 +188,7 @@ grub_file_is_not_garbage ()
       *.dpkg-*) return 1 ;; # debian dpkg
       *.rpmsave|*.rpmnew) return 1 ;;
       README*|*/README*)  return 1 ;; # documentation
+      *.sig) return 1 ;; # signatures
     esac
   else
     return 1