[media] dvb-usb-v2: get rid of warning: no previous prototype

drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:227:5: warning: no previous prototype for 'dvb_usbv2_adapter_stream_init' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:239:5: warning: no previous prototype for 'dvb_usbv2_adapter_stream_exit' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:372:5: warning: no previous prototype for 'dvb_usbv2_adapter_dvb_init' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:444:5: warning: no previous prototype for 'dvb_usbv2_adapter_dvb_exit' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:460:5: warning: no previous prototype for 'dvb_usbv2_device_power_ctrl' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:557:5: warning: no previous prototype for 'dvb_usbv2_adapter_frontend_init' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/dvb_usb_core.c:626:5: warning: no previous prototype for 'dvb_usbv2_adapter_frontend_exit' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/usb_urb.c:115:5: warning: no previous prototype for 'usb_urb_free_urbs' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/usb_urb.c:208:5: warning: no previous prototype for 'usb_free_stream_buffers' [-Wmissing-prototypes]
drivers/media/usb/dvb-usb-v2/usb_urb.c:226:5: warning: no previous prototype for 'usb_alloc_stream_buffers' [-Wmissing-prototypes]

Cc: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>