make spatial svc an experiment

disabled by default, enable with:
--enable-experimental --enable-spatial-svc

this disables vp9_spatial_svc_encoder and svc_test, further work is
needed to remove internal lib references

Change-Id: I6a487ecbf07eb98843a99d96e17f08f960b63088
diff --git a/configure b/configure
index bd95056..cc4ca02 100755
--- a/configure
+++ b/configure
@@ -265,8 +265,9 @@
     unistd_h
 "
 EXPERIMENT_LIST="
-    multiple_arf
     alpha
+    multiple_arf
+    spatial_svc
 "
 CONFIG_LIST="
     external_build