isdn: fix netjet/isdnhdlc build errors

Commit cb3824ba didn't fix this problem.

Fix build errors in netjet, using isdnhdlc module:

drivers/built-in.o: In function `mode_tiger':
netjet.c:(.text+0x1ca0c7): undefined reference to `isdnhdlc_rcv_init'
netjet.c:(.text+0x1ca0d4): undefined reference to `isdnhdlc_out_init'
drivers/built-in.o: In function `fill_dma':
netjet.c:(.text+0x1ca2bd): undefined reference to `isdnhdlc_encode'
drivers/built-in.o: In function `read_dma':
netjet.c:(.text+0x1ca614): undefined reference to `isdnhdlc_decode'
drivers/built-in.o: In function `nj_irq':
netjet.c:(.text+0x1cb07a): undefined reference to `isdnhdlc_encode'

drivers/built-in.o: In function `isdnhdlc_decode':
(.text+0x1c2088): undefined reference to `crc_ccitt_table'
drivers/built-in.o: In function `isdnhdlc_encode':
(.text+0x1c2339): undefined reference to `crc_ccitt_table'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>