clang-tidy: Add performance-type-promotion-in-math-fn check

https://clang.llvm.org/extra/clang-tidy/checks/performance/type-promotion-in-math-fn.html

src/qt/trafficgraphwidget.cpp

@@ -68,8 +68,8 @@ void TrafficGraphWidget::paintEvent(QPaintEvent *)
     painter.drawLine(XMARGIN, YMARGIN + h, width() - XMARGIN, YMARGIN + h);
 
     // decide what order of magnitude we are
-    int base = floor(log10(fMax));
-    float val = pow(10.0f, base);
+    int base = std::floor(std::log10(fMax));
+    float val = std::pow(10.0f, base);
 
     const QString units = tr("kB/s");
     const float yMarginText = 2.0;