[media] lgdt3306a: Use IS_ENABLED() for attach function

Simplify the check if CONFIG_DVB_LGDT3306A is enabled, use the
IS_ENABLED() macro, just like the other frontend modules.
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-frontends/lgdt3306a.h

@@ -63,9 +63,7 @@ struct lgdt3306a_config {
 	int  xtalMHz;
 };
 
-#if defined(CONFIG_DVB_LGDT3306A) || (defined(CONFIG_DVB_LGDT3306A_MODULE) && \
-				     defined(MODULE))
-extern
+#if IS_ENABLED(CONFIG_DVB_LGDT3306A)
 struct dvb_frontend *lgdt3306a_attach(const struct lgdt3306a_config *config,
 				      struct i2c_adapter *i2c_adap);
 #else