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](bitcoin/bitcoin#7982 (comment))
and is unnecessary for us anyhow.

src/tinyformat.h

@@ -113,7 +113,7 @@ namespace tfm = tinyformat;
 
 // Define for C++11 variadic templates which make the code shorter & more
 // general.  If you don't define this, C++11 support is autodetected below.
-// #define TINYFORMAT_USE_VARIADIC_TEMPLATES
+#define TINYFORMAT_USE_VARIADIC_TEMPLATES
 
 
 //------------------------------------------------------------------------------