Install the same version of linux-compiler-gcc-* as the kernel we install.

While preparing Tails 2.6 it was discovered that our last minute bump of
the 'debian' snapshot (to get Jessie 8.6) cause linux-compiler-gcc-* to
be updated to a version for another Linux version than what we install,
resulting in the tagged snapshot being impossible to create since it
would require two different linux source packages (which is not
supported).