vlc extension: fix long-standing compilation warning

Headers/Public/VLCExtension.h

@@ -21,6 +21,7 @@
  *****************************************************************************/
 
 #import <Cocoa/Cocoa.h>
+#import <vlc_extensions.h>
 
 @interface VLCExtension : NSObject
 

Sources/VLCExtension.m

@@ -21,7 +21,6 @@
  *****************************************************************************/
 
 #import "VLCExtension.h"
-#import <vlc_extensions.h>
 
 @interface VLCExtension ()
 {
@@ -35,6 +34,11 @@
     return [NSString stringWithFormat:@"VLC Extension %@", [self name]];
 }
 
+- (instancetype)init
+{
+    return [self initWithInstance:NULL];
+}
+
 - (instancetype)initWithInstance:(struct extension_t *)instance
 {
     self = [super init];