tinyformat: force USE_VARIADIC_TEMPLATES
tinyformat: force USE_VARIADIC_TEMPLATES

Now that we started using c++11, force use of variadic templates.

The autodetection may be wonky on some compilers, see discussion
[here](https://github.com/bitcoin/bitcoin/pull/7982#issuecomment-216222357)
and is unnecessary for us anyhow.

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