Add missing include file stdlib.h for abs(int)

This file was included (in math.h or elsewhere) on Linux and latest
macOS versions but maybe not in earlier macOS versions or with other
configuration options.

src/drivers/PostScript/Fl_PostScript_image.cxx

@@ -20,6 +20,7 @@
 #include <stdio.h>
 #include <math.h>
 #include <string.h>
+#include <stdlib.h>  // abs(int)
 
 #include <FL/Fl_PostScript.H>
 #include "Fl_PostScript_Graphics_Driver.H"