add dosfsutils to dependencies

[Danny Kulchinsky]

Makefile:33: recipe for target 'c.vmdk' failed
make: [c.vmdk] Error 1 (ignored)
/sbin/mkfs.vfat -C c.img -F 32 "$(du -s c | awk '{print $1 + 10000}')"
/bin/sh: 1: /sbin/mkfs.vfat: not found
Makefile:33: recipe for target 'c.vmdk' failed
make: *** [c.vmdk] Error 127

On Debian, mkfs.vfat is provided by dosfsutils

Thanks!