wgl: relocate wgl code to state_trackers/wgl

Similar to the GLX state trackers for DRI and xlib.
diff --git a/src/gallium/SConscript b/src/gallium/SConscript
index 6a3e7e7..9e4596a 100644
--- a/src/gallium/SConscript
+++ b/src/gallium/SConscript
@@ -27,3 +27,6 @@
 	SConscript(os.path.join('drivers', driver, 'SConscript'))
 
 SConscript('state_trackers/python/SConscript')
+
+if platform == 'windows':
+	SConscript('state_trackers/wgl/SConscript')