fix wrong unique_lock usage

clang warns about the wrong usage:
```
```
xmrstak/net/jpsock.cpp:232:30: warning: parentheses were disambiguated as redundant parentheses around declaration of variable named 'job_mutex' [-Wvexing-parse]
        std::unique_lock<std::mutex>(job_mutex);
                                    ^~~~~~~~~~~
xmrstak/net/jpsock.cpp:232:30: note: add a variable name to declare a 'std::unique_lock<std::mutex>' initialized with 'job_mutex'
        std::unique_lock<std::mutex>(job_mutex);
```