Restore //:protoc_lib target.

This was removed in
cbd1adc6cf,
but it’s still present in the released versions of Protocol Buffers, and since
it’s a public target it’s part of the public API.

PiperOrigin-RevId: 495828746

BUILD.bazel

@@ -158,6 +158,12 @@ internal_ruby_proto_library(
 # Protocol Buffers Compiler
 ################################################################################
 
+alias(
+    name = "protoc_lib",
+    actual = "//src/google/protobuf/compiler:protoc_lib",
+    visibility = ["//visibility:public"],
+)
+
 cc_binary(
     name = "protoc",
     copts = COPTS,