vp9_encoder: add missing vpx_clear_system_state

fixes sse-only build

Change-Id: I4591f5be6773464238744a41d6901cf0e903b5bd

vp9/encoder/vp9_encoder.c

diff --git a/vp9/encoder/vp9_encoder.c b/vp9/encoder/vp9_encoder.c
index 0680550..5261feb 100644
--- a/vp9/encoder/vp9_encoder.c
+++ b/vp9/encoder/vp9_encoder.c
@@ -4547,6 +4547,7 @@
         cpi->svc.spatial_layer_to_encode = 0;
     }
   }
+  vpx_clear_system_state();
   return 0;
 }