scull: Makefile: Declare LDDINC variable earlier

Move the definition of LDDINC earlier into the file. This prevents the
following compile time error: implicit declaration of function ‘KBUILD_STR’.
Don't use create_proc_entry() and create_proc_read_entry()

Don't use create_proc_entry() and create_proc_read_entry() as they
are deprecated, but rather use proc_create() and proc_create_data()
respectively and seq_file instead.