Move av1_indices_from_tree() to common code space.

Move the av1_indices_from_tree() function from av1/encoder/treewriter.c
 to aom_dsp/prob.c so that it can be used by both the encoder and
 the decoder.

Change-Id: Ie43c599f425c3503b1ff93f0c77b5033a05b1bb4
diff --git a/aom_dsp/prob.h b/aom_dsp/prob.h
index f0ea01b..3ae34b8 100644
--- a/aom_dsp/prob.h
+++ b/aom_dsp/prob.h
@@ -127,6 +127,9 @@
       }                                                \
     }                                                  \
   } while (0)
+
+void av1_indices_from_tree(int *ind, int *inv, int len,
+                           const aom_tree_index *tree);
 #endif
 
 DECLARE_ALIGNED(16, extern const uint8_t, aom_norm[256]);