spl build failure on a clean install of Wily

SRU request, Wily.

[IMPACT]
Installing ZFS on a clean machine that does not have libc-dev installed will end in a DKMS build failure on the Solaris Porting Layer module.

[TEST CASE]
Do a clean install of Ubuntu Wily server
sudo apt-get update && sudo apt-get dist-upgrade
sudo apt-get install zfsutils-linux

Without the fix, this breaks. With the fix SPL builds OK and ZFS installs.

[REGRESSION POTENTIAL]
Minimal, this just adds one dependency in for spl-dkms, and this package will ultimately get installed later on during the zfsutils-linux installation. This fix causes it to be installed earlier on during the installation.

----------------

On a totally clean server install of Wily installing zfsutils-linux ends up with a build failure of spl, typically:

Setting up zfs-dkms (0.6.4.2-0ubuntu1.2) ...
Loading new zfs-0.6.4.2 DKMS files...
First Installation: checking all kernels...
Building only for 4.2.0-25-generic
Building initial module for 4.2.0-25-generic
configure: error:
        *** Please make sure the kmod spl devel <kernel> package for your
        *** distribution is installed then try again. If that fails you
        *** can specify the location of the spl objects with the
        *** '--with-spl-obj=PATH' option.
Error! Bad return status for module build on kernel: 4.2.0-25-generic (x86_64)
Consult /var/lib/dkms/zfs/0.6.4.2/build/make.log for more information.

The issue is that the configure script in SPL is building a simple test program but the linking fails because libc-dev is not yet installed even though gcc is. The libc-dev package gets installed a little while later.

A fix is to modify spl-linux: debian/control and add in the libc-dev dependency on spl-dkms to ensure this package is already installed before SPL runs the test.

Package: spl-dkms
Depends: ${misc:Depends}, libc-dev, dkms (>= 2.2.0.2), file, lsb-release
Recommends: spl

Normally, dkms driver builds just build against the kernel without such pre-checks, so we don't hit this with normal dkms packages. Note that this issue does not seem to bite a lot of users probably because libc-dev is already installed.