ThinLTO Linux ToT bot: fix a typo, cfi -> lto.

I have mistyped the mixin name. Sorry.

BUG=645295
TBR=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2331423007
Cr-Commit-Position: refs/heads/master@{#418806}

tools/mb/mb_config.pyl

@@ -2463,7 +2463,7 @@
 
     'thin_lto': {
       'gn_args': 'use_thin_lto=true',
-      'mixins': ['cfi'],
+      'mixins': ['lto'],
     },
 
     'tsan': {