Make framework:allocator target public.

There is already an alias that is public in tensorflow/core.
This is just encouraging the use of the alias.

PiperOrigin-RevId: 336347937
Change-Id: Ie6b248083a5ac5eb4582938f65d001b2de080aea

tensorflow/core/framework/BUILD

@@ -454,7 +454,7 @@ cc_library(
         "allocator.h",
     ],
     features = ["parse_headers"],
-    visibility = ["//tensorflow/core:__subpackages__"],
+    visibility = ["//visibility:public"],
     deps = [
         ":numeric_types",
         ":type_traits",