Fix missing <math.h> header in qrenderdoc for Arch/clang++

Add the <math.h> header to some of the source files for the target
qrenderdoc. The build would not have succeded without these headers
declaration under Arch Linux/clang++.

qrenderdoc/Widgets/CustomPaintWidget.cpp

@@ -23,6 +23,7 @@
  ******************************************************************************/
 
 #include "CustomPaintWidget.h"
+#include <math.h>
 #include <QPainter>
 #include "Code/CaptureContext.h"
 #include "renderdoc_replay.h"