macosx: Fix trying to load playlist as an SD

The sdtype of VLCSourceListItem was not correctly initialized to -1,
which caused the macosx interface trying to load the playlist like am
SD module, which does not work.

modules/gui/macosx/VLCSourceListItem.m

@@ -25,4 +25,14 @@
 
 @implementation VLCSourceListItem
 
+- (id)init
+{
+    self = [super init];
+    if (self) {
+        _sdtype = -1; // No SD type set
+    }
+
+    return self;
+}
+
 @end