[media] Kconfig: rtl2832_sdr must depend on USB

Fixes error:
[next:master 7435/8702] ERROR: "usb_alloc_urb
[drivers/media/dvb-frontends/rtl2832_sdr.ko] undefined!

rtl2832_sdr driver implements own USB streaming for SDR data.
Logically that functionality belongs to USB interface driver, but
currently it is implemented here.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/dvb-frontends/Kconfig

@@ -448,7 +448,7 @@ config DVB_RTL2832
 
 config DVB_RTL2832_SDR
 	tristate "Realtek RTL2832 SDR"
-	depends on DVB_CORE && I2C && I2C_MUX && VIDEO_V4L2 && MEDIA_SDR_SUPPORT
+	depends on DVB_CORE && I2C && I2C_MUX && VIDEO_V4L2 && MEDIA_SDR_SUPPORT && USB
 	select DVB_RTL2832
 	select VIDEOBUF2_VMALLOC
 	default m if !MEDIA_SUBDRV_AUTOSELECT