[media] drx39xxj.h: Fix undefined reference to attach function

As reported by the kbuild test robot <fengguang.wu@intel.com>:

drivers/built-in.o: In function `em28xx_dvb_init':
    em28xx-dvb.c:(.text+0x876f2c): undefined reference to `drx39xxj_attach'

That happens when CONFIG_VIDEO_EM28XX_DVB is selected, and neither
CONFIG_MEDIA_SUBDRV_AUTOSELECT or DVB_DRX39XYJ is selected.
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/dvb-frontends/drx39xyj/drx39xxj.h

@@ -34,6 +34,12 @@ struct drx39xxj_state {
 	const struct firmware *fw;
 };
 
+#if IS_ENABLED(CONFIG_DVB_DRX39XYJ)
 struct dvb_frontend *drx39xxj_attach(struct i2c_adapter *i2c);
+#else
+static inline struct dvb_frontend *drx39xxj_attach(struct i2c_adapter *i2c) {
+	return NULL;
+};
+#endif
 
 #endif /* DVB_DUMMY_FE_H */