Re-add mtools to Depends as they are just suggests of syslinux package

Quoting http://bts.grml.org/grml/issue905:

  I'm using grml2usb (0.9.25) from grml repo on a Debian Squeeze system. grml2usb
  fails with the message

  Installing syslinux as bootloader
  sh: mcopy: not found
  syslinux: failed to create ldlinux.sys

  That's because syslinux package in Debian Squezze just 'suggests' mtools·
  package. I filed a Bug in Debian BTS requesting syslinux at least to·
  recommend mtools package but it was closed... (Bug#594331)

Thanks: Michael Gissing
Closes: issue905