zlib build fix. (#333)

* zlib build fix.

grpc renamed @com_github_madler_zlib//:z to @zlib//:zlib in
grpc/grpc@79f7abb

Author: g-easy

tools/zlib/BUILD

@@ -12,10 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# @com_github_madler_zlib comes from grpc_deps in the outer workspace.
+# @zlib comes from grpc_deps in the outer workspace.
 
 cc_library(
     name = "z",
     visibility = ["//visibility:public"],
-    deps = ["@com_github_madler_zlib//:z"],
+    deps = ["@zlib"],
 )