Merge pull request #14011 from apolcyn/avoid_ares_on_windows

Correct a usage of select in grpc_build_system

bazel/grpc_build_system.bzl

@@ -30,12 +30,12 @@ def _get_external_deps(external_deps):
   ret = []
   for dep in external_deps:
     if dep == "nanopb":
-      ret.append("//third_party/nanopb")
+      ret += ["//third_party/nanopb"]
     elif dep == "cares":
       ret += select({"//:grpc_no_ares": [],
                      "//conditions:default": ["//external:cares"],})
     else:
-      ret.append("//external:" + dep)
+      ret += ["//external:" + dep]
   return ret
 
 def _maybe_update_cc_library_hdrs(hdrs):