Fix compilation warning introduced in #246

QDebug include was missing in main.cpp, and the qDebug() call was using a concatenation of variables for the format string, causing a warning:
```
/cockatrice/src/main.cpp:162:16: warning: format string is not a string
literal (potentially insecure) [-Wformat-security]
        qDebug("Could not create " +
settingsCache->getPicsPath().toUtf8() + "/CUSTOM. Will fall back on
default card images.");
```