SW: add syslinux-common to GRMLBASE

Otherwise the boot related modules might be missing:

| * Missing addon file: "chain.c32"                                        [ !! ]
| * Missing addon file: "hdt.c32"                                          [ !! ]
| * Missing addon file: "mboot.c32"                                        [ !! ]
| * Missing addon file: "menu.c32"                                         [ !! ]

This is happening on grml-small where syslinux-common isn't
present by default because it's just a Recommends but not a Depends.
On grml-full we ship syslinux-common by default though because
it's a dependency of grub-imageboot.