Fix dev build with Qt 5.15 on MacOS

[riordant]

PR intention

master build failing on MacOS in Qt:

qt/trafficgraphwidget.cpp:53:13: error: member access into incomplete type 'QPainterPath'
        path.moveTo(x, YMARGIN + h);
            ^
/usr/local/Cellar/qt/5.15.0/lib/QtGui.framework/Headers/qmatrix.h:54:7: note: forward declaration of 'QPainterPath'
class QPainterPath;
      ^
qt/trafficgraphwidget.cpp:57:17: error: member access into incomplete type 'QPainterPath'
            path.lineTo(x, y);
                ^
/usr/local/Cellar/qt/5.15.0/lib/QtGui.framework/Headers/qmatrix.h:54:7: note: forward declaration of 'QPainterPath'
class QPainterPath;
      ^
qt/trafficgraphwidget.cpp:59:13: error: member access into incomplete type 'QPainterPath'
        path.lineTo(x, YMARGIN + h);
            ^
/usr/local/Cellar/qt/5.15.0/lib/QtGui.framework/Headers/qmatrix.h:54:7: note: forward declaration of 'QPainterPath'
class QPainterPath;
      ^
qt/trafficgraphwidget.cpp:106:22: error: variable has incomplete type 'QPainterPath'
        QPainterPath p;
                     ^
/usr/local/Cellar/qt/5.15.0/lib/QtGui.framework/Headers/qmatrix.h:54:7: note: forward declaration of 'QPainterPath'
class QPainterPath;
      ^
qt/trafficgraphwidget.cpp:113:22: error: variable has incomplete type 'QPainterPath'
        QPainterPath p;
                     ^
/usr/local/Cellar/qt/5.15.0/lib/QtGui.framework/Headers/qmatrix.h:54:7: note: forward declaration of 'QPainterPath'
class QPainterPath;

Appears related to latest release of Qt5.15 (Nheko-Reborn/nheko@43d2ebc) .

Code changes brief

Just include QPainterPath in the file with the failure