Added Bazel genrule for generating well_known_types_embed.cc

In pull request protocolbuffers#2517 I made this change for the CMake and autotools
builds but forgot to do it for the Bazel build.

BUILD

@@ -227,6 +227,24 @@ cc_proto_library(
 # Protocol Buffers Compiler
 ################################################################################
 
+cc_binary(
+    name = "js_embed",
+    srcs = ["src/google/protobuf/compiler/js/embed.cc"],
+    visibility = ["//visibility:public"],
+)
+
+genrule(
+    name = "generate_js_well_known_types_embed",
+    srcs = [
+        "src/google/protobuf/compiler/js/well_known_types/any.js",
+        "src/google/protobuf/compiler/js/well_known_types/struct.js",
+        "src/google/protobuf/compiler/js/well_known_types/timestamp.js",
+    ],
+    outs = ["src/google/protobuf/compiler/js/well_known_types_embed.cc"],
+    cmd = "$(location :js_embed) $(SRCS) > $@",
+    tools = [":js_embed"],
+)
+
 cc_library(
     name = "protoc_lib",
     srcs = [