[EventEngine] Adjust visibility of event_engine_base_hdrs and //src/core:useful targets

PiperOrigin-RevId: 594060115

BUILD

@@ -1274,6 +1274,7 @@ grpc_cc_library(
     tags = [
         "nofixdeps",
     ],
+    visibility = ["@grpc:event_engine_base_hdrs"],
     deps = [
         "channel_arg_names",
         "gpr",

bazel/grpc_build_system.bzl

@@ -120,6 +120,8 @@ def _update_visibility(visibility):
         "xds": PRIVATE,
         "xds_client_core": PRIVATE,
         "grpc_python_observability": PRIVATE,
+        "event_engine_base_hdrs": PRIVATE,
+        "useful": PRIVATE,
     }
     final_visibility = []
     for rule in visibility:

src/core/BUILD

@@ -155,6 +155,7 @@ grpc_cc_library(
         "absl/types:variant",
     ],
     language = "c++",
+    visibility = ["@grpc:useful"],
     deps = ["//:gpr_platform"],
 )