Fix compilation error on some versions of GCC

src/libexpr/primops.cc:42:8: error: looser throw specifier for 'virtual nix::InvalidPathError::~InvalidPathError()'
src/libexpr/nixexpr.hh:12:1: error:   overriding 'virtual nix::EvalError::~EvalError() noexcept (true)'

http://hydra.nixos.org/build/12385750

src/libexpr/primops.cc

@@ -44,6 +44,7 @@ struct InvalidPathError : EvalError
     Path path;
     InvalidPathError(const Path & path) :
         EvalError(format("path `%1%' is not valid") % path), path(path) {};
+    ~InvalidPathError() throw () { };
 };