gallium/targets: Break haiku state_tracker out to own directory

Ack'ed by Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/src/gallium/SConscript b/src/gallium/SConscript
index 98d017e..977e3fb 100644
--- a/src/gallium/SConscript
+++ b/src/gallium/SConscript
@@ -85,6 +85,7 @@
 
     if env['platform'] == 'haiku':
         SConscript([
+            'state_trackers/hgl/SConscript',
             'targets/haiku-softpipe/SConscript',
         ])