systemlibs: icu: update unbundle

//third_party/icu/data was added which depends on a new icu target that
was missing in the unbundled BUILD file.

Signed-off-by: Jason Zaman <jason@perfinion.com>

third_party/icu/BUILD.system

@@ -1,13 +1,19 @@
+package(
+    default_visibility = ["//visibility:public"],
+)
+
 licenses(["notice"])  # Apache 2.0
 
 filegroup(
     name = "icu4c/LICENSE",
-    visibility = ["//visibility:public"],
 )
 
 filegroup(
     name = "icu4j/main/shared/licenses/LICENSE",
-    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "headers",
 )
 
 cc_library(
@@ -15,7 +21,6 @@ cc_library(
     deps = [
         ":icuuc",
     ],
-    visibility = ["//visibility:public"],
 )
 
 cc_library(