Temporarily relocate av1_resize_frame420 export.

Move it to aom/exports_com to clear some shared library
build issues. It will be moved to where it belongs in a
follow up (av1/exports_com is where it should be).

BUG=aomedia:2072

Change-Id: I54448dd7068563005043de34f24bdc3312d30591
diff --git a/aom/exports_com b/aom/exports_com
index 4d06b76..c6da23b 100644
--- a/aom/exports_com
+++ b/aom/exports_com
@@ -22,3 +22,4 @@
 text aom_uleb_encode
 text aom_uleb_encode_fixed_size
 text aom_uleb_size_in_bytes
+text av1_resize_frame420
diff --git a/av1/exports_test b/av1/exports_test
index 755dc90..dab3775 100644
--- a/av1/exports_test
+++ b/av1/exports_test
@@ -1,3 +1,2 @@
 text av1_get_fwd_txfm_cfg
 text av1_rtcd
-text av1_resize_frame420