Fix for warning: deprecated conversion from string constant to 'char*'

Index: src/ScreenResource.cc
--- src/ScreenResource.cc.orig
+++ src/ScreenResource.cc
@@ -32,6 +32,7 @@
 #include <Resource.hh>
 
 #include <assert.h>
+#include <string.h>
 
 
 static const int iconify_width  = 9;
@@ -61,7 +62,7 @@ static const unsigned char close_bits[] =
 
 void ScreenResource::save(bt::Resource& res, BScreen* screen) {
   char rc_string[128];
-  char *placement = (char *) 0;
+  const char *placement = NULL;
   unsigned int number = screen->screenNumber();
 
   switch (_slitOptions.placement) {