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

diff --git a/build_defs.bzl b/build_defs.bzl
index 1dc3de9..8afac41 100644
--- a/build_defs.bzl
+++ b/build_defs.bzl
@@ -212,6 +212,7 @@
             default = "@com_google_emboss//runtime/cpp:cpp_utils",
         ),
     },
+    toolchains = ["@bazel_tools//tools/cpp:toolchain_type"],
 )
 
 def _cc_emboss_library_impl(ctx):