Fix emboss_cpp_library Bazel rule (#104)

Add `toolchains` parameter to _cc_emboss_aspect rule to fix
find_cpp_toolchain error.

Fixes #103

build_defs.bzl

@@ -212,6 +212,7 @@ _cc_emboss_aspect = aspect(
  default = "@com_google_emboss//runtime/cpp:cpp_utils",
  ),
  },
+ toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
 )
 
 def _cc_emboss_library_impl(ctx):