don't pick up /usr/include/db.h

Index: src/libicalss/icalbdbsetimpl.h
--- src/libicalss/icalbdbsetimpl.h.orig
+++ src/libicalss/icalbdbsetimpl.h
@@ -19,7 +19,7 @@
 #define ICALBDBSETIMPL_H
 
 #include "icalset.h"
-#include <db.h>
+#include <db4/db.h>
 
 /* This definition is in its own file so it can be kept out of the
    main header file, but used by "friend classes" like icaldirset*/