TextEditor: Fix compile on macOS

Change-Id: I78929b2d3f6e7803879e63ba5611e5210cc53f90
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/generichighlighter/highlighter.cpp

@@ -38,6 +38,8 @@
 
 #include <QCoreApplication>
 
+#include <cmath>
+
 using namespace TextEditor;
 using namespace Internal;