media: dvbdev: fix building on ia64

Not sure why, but, on ia64, with Linaro's gcc 7.3 compiler,
using #ifdef (CONFIG_I2C) is not OK.

So, replace it by IS_ENABLED(CONFIG_I2C), in order to fix the
builds there.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/dvb-core/dvbdev.c

@@ -942,7 +942,7 @@ int dvb_usercopy(struct file *file,
 	return err;
 }
 
-#ifdef CONFIG_I2C
+#if IS_ENABLED(CONFIG_I2C)
 struct i2c_client *dvb_module_probe(const char *module_name,
 				    const char *name,
 				    struct i2c_adapter *adap,

include/media/dvbdev.h

@@ -358,7 +358,7 @@ long dvb_generic_ioctl(struct file *file,
 int dvb_usercopy(struct file *file, unsigned int cmd, unsigned long arg,
 		 int (*func)(struct file *file, unsigned int cmd, void *arg));
 
-#ifdef CONFIG_I2C
+#if IS_ENABLED(CONFIG_I2C)
 
 struct i2c_adapter;
 struct i2c_client;