Limit the number of targets the VNC platform plugin is be built for

The VNC platform plugin is a port from the 4.8 code, which only
supported embedded Linux.  Right now that is the only platform this is
tested with so unless there is a need for other platforms to be hosting
applications over VNC we should limit it to only being built on unix
platforms that are not Android, or Darwin.

Change-Id: I49407669ee8ca6e9e3ed95b5bb350c7730839cf8
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/platforms.pro

@@ -2,7 +2,7 @@ TEMPLATE = subdirs
 
 android: SUBDIRS += android
 
-!android: SUBDIRS += minimal vnc
+!android: SUBDIRS += minimal
 
 !android:if(!win32|contains(QT_CONFIG, freetype)): SUBDIRS += offscreen
 
@@ -35,6 +35,8 @@ contains(QT_CONFIG, directfb) {
 
 contains(QT_CONFIG, linuxfb): SUBDIRS += linuxfb
 
+unix:!android:!darwin: SUBDIRS += vnc
+
 freebsd {
     SUBDIRS += bsdfb
 }