fix friend declaration with argument, violate spec

Build with lates clang++ 5.1 on OSX 10.9, the following error is reported.
The code in question violate C++ 11 spec, and clang is enforcing that rule.

C++11: §8.3.6.4 [dcl.fct.default]
"If a friend declaration specifies a default argument expression,
that declaration shall be a definition and shall be the only declaration
of the function or function template in the translation unit."