Minimize and fix include statements of Fl_Int_Vector

- replace <FL/Fl.H> with <FL/Fl_Export.H> in public header
- add missing <string.h> in implementation

FL/Fl_Int_Vector.H

@@ -2,7 +2,7 @@
 // An STL-ish vector without templates for the Fast Light Tool Kit (FLTK).
 //
 // Copyright 2002 by Greg Ercolano.
-// Copyright 2022 by Bill Spitzak and others.
+// Copyright 2022-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -18,7 +18,7 @@
 #ifndef Fl_Int_Vector_H
 #define Fl_Int_Vector_H
 
-#include <FL/Fl.H>
+#include <FL/Fl_Export.H>
 
 /** \file FL/Fl_Int_Vector.H
   An STL-ish vector implemented without templates.

src/Fl_Int_Vector.cxx

@@ -17,6 +17,7 @@
 
 #include <FL/Fl_Int_Vector.H>
 #include <stdlib.h>
+#include <string.h>
 
 /**
   Make a copy of another array.