Add e2fsprogs to Depends

Debian wants to make removing e2fsprogs from installations
possible.  Once e2fsprogs drops the "Essential: yes" flag
e2fsprogs's mkfs.ext4 binary won't necessarily be available any
longer, while we default to usage of mkfs.ext4 in
grml-debootstrap.

Thanks: Helmut Grohne <helmut@subdivi.de> for the bugreport
Closes: #887224