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

Index: src/libicalss/icalbdbset.h
--- src/libicalss/icalbdbset.h.orig
+++ src/libicalss/icalbdbset.h
@@ -21,7 +21,7 @@
 #include "libical_icalss_export.h"
 #include "icalset.h"
 
-#include <db.h>
+#include <db4/db.h>
 
 typedef struct icalbdbset_impl icalbdbset;