Scull updated for 3.10

Hello Javier. I'm learning kernel programming with the ldd3 book and I'm using your updated example code. Unfortunately (or not ;) many of the examples don't work with recent kernels and I'm trying to fix them as I advance my reading.

This pull request adds the necessary changes to compile the scull driver.
 - The first change is in the Makefile. Moving the declaration of LDDINC earlier solves the compile problem.
 - The second change has to do with the proc filesystem. The old interface to create proc entries has been removed in kernel 3.10. I've updated the interface calls and it's using seq_files now.