fix(opensmt/minsat): fix default argument in template friend function problem

Clang++-3.4 is starting to enforce the following item of C++11 standard,
thus it's making lean not compiling:

It's illegal in 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."
fix(opensmt/minsat): fix default argument in template friend function problem