Only show sendPrimary on X11 platforms

Primary selection is inherently a X11 concept so there is no point
showing the settings related to it on other platforms.
diff --git a/vncviewer/parameters.h b/vncviewer/parameters.h
index 0de61d2..e4378c8 100644
--- a/vncviewer/parameters.h
+++ b/vncviewer/parameters.h
@@ -52,7 +52,9 @@
 
 extern rfb::BoolParameter acceptClipboard;
 extern rfb::BoolParameter sendClipboard;
+#if !defined(WIN32) && !defined(__APPLE__)
 extern rfb::BoolParameter sendPrimary;
+#endif
 
 extern rfb::StringParameter menuKey;