net: qmi_wwan: fix build error due to cdc-wdm dependecy

Fixes:

drivers/built-in.o: In function `qmi_wwan_bind_shared':
qmi_wwan.c:(.text+0x25b686): undefined reference to `usb_cdc_wdm_register'
make[1]: *** [.tmp_vmlinux1] Error 1

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>