Fix VS build failure

Making function implementation match the declaration.

Change-Id: I94eb4560c57406e0fa433e17f1eee6aa64c39181

av1/encoder/context_tree.c

diff --git a/av1/encoder/context_tree.c b/av1/encoder/context_tree.c
index f0e5f8a..566576e 100644
--- a/av1/encoder/context_tree.c
+++ b/av1/encoder/context_tree.c
@@ -65,7 +65,8 @@
   }
 }
 
-PICK_MODE_CONTEXT *av1_alloc_pmc(const struct AV1_COMP *cpi, BLOCK_SIZE bsize,
+PICK_MODE_CONTEXT *av1_alloc_pmc(const struct AV1_COMP *const cpi,
+                                 BLOCK_SIZE bsize,
                                  PC_TREE_SHARED_BUFFERS *shared_bufs) {
   PICK_MODE_CONTEXT *ctx = NULL;
   const AV1_COMMON *const cm = &cpi->common;