gcc and qt-5.12.0 compilation error fix (#171)

Compililation with gcc (any version) and Qt 5.12.0 (important) fails with error:
```
Core/private/sysapiloader_p.h:59:26: error: no matching function for call to ‘QMutex::QMutex()’
     static inline QMutex m_mutex;
                          ^~~~~~~
include/QtCore/qmutex.h:165:5: note: candidate: ‘QMutex::QMutex(const QMutex&)’ <deleted>
     Q_DISABLE_COPY(QMutex)
     ^~~~~~~~~~~~~~
```

`QMutex` in Qt 5.12.0 has explicit (!) one-parameter constructor with a
default value: `QMutex(QMutex::RecursionMode mode = NonRecursive)` and no
default constructor. In this case declaring static inline member without
initializer causes the above error.

include/FramelessHelper/Core/private/sysapiloader_p.h

@@ -56,7 +56,7 @@ class FRAMELESSHELPER_CORE_API SysApiLoader : public QObject
     }
 
 private:
-    static inline QMutex m_mutex;
+    static inline QMutex m_mutex{};
     static inline QHash<QString, std::optional<QFunctionPointer>> m_functionCache = {};
 };