Fix harmless warning about missing return type in initWithSession:

The return type defaults to "id" anyhow, but it's better to specify it
explicitly.

src/osx/webrequest_urlsession.mm

@@ -34,7 +34,7 @@
 
 @implementation wxWebSessionDelegate
 
-- initWithSession:(wxWebSessionURLSession*)session
+- (id)initWithSession:(wxWebSessionURLSession*)session
 {
     m_session = session;
     m_requests = [[NSMapTable weakToStrongObjectsMapTable] retain];