NSSharingService: define class

Define the NSSharingService class as an unimplemented stub for
compatibility purposes.

Source/NSSharingService.m

@@ -0,0 +1,33 @@
+/** NSSharingService.m
+
+   Copyright <copy>(C) 2017 Free Software Foundation, Inc.</copy>
+
+   Author: Daniel Ferreira <dtf@stanford.edu>
+   Date: 2017
+
+   This file is part of the GNUstep GUI Library.
+
+   This library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2 of the License, or (at your option) any later version.
+
+   This library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with this library; see the file COPYING.LIB.
+   If not, see <http://www.gnu.org/licenses/> or write to the 
+   Free Software Foundation, 51 Franklin Street, Fifth Floor, 
+   Boston, MA 02110-1301, USA.
+*/
+
+#import <AppKit/NSSharingService.h>
+
+@implementation NSSharingService
+@end
+
+@implementation NSSharingServicePicker
+@end