icalstrdup: fixed compile issue in EDS backend

config.h wasn't included early enough, so icalstrdup()
wasn't defined although needed.

src/syncevo/icalstrdup.h

@@ -20,6 +20,10 @@
 #ifndef ICALSTRDUP_H
 #define ICALSTRDUP_H
 
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
 #ifdef ENABLE_ICAL
 
 #ifndef HANDLE_LIBICAL_MEMORY