Added bazel_skylib dependency in preparation for protobuf update

Protobuf 3.7.0 will depend on bazel_skylib, so gRPC needs to add it as a
dependency in its own workspace.

bazel/grpc_deps.bzl

@@ -185,6 +185,14 @@ def grpc_deps():
             sha256 = "ee854b5de299138c1f4a2edb5573d22b21d975acfc7aa938f36d30b49ef97498",
         )
 
+    if "bazel_skylib" not in native.existing_rules():
+        http_archive(
+            name = "bazel_skylib",
+            sha256 = "ba5d15ca230efca96320085d8e4d58da826d1f81b444ef8afccd8b23e0799b52",
+            strip_prefix = "bazel-skylib-f83cb8dd6f5658bc574ccd873e25197055265d1c",
+            urls = ["https://github.com/bazelbuild/bazel-skylib/archive/f83cb8dd6f5658bc574ccd873e25197055265d1c.tar.gz"],
+        )
+
     if "io_opencensus_cpp" not in native.existing_rules():
         http_archive(
             name = "io_opencensus_cpp",