Include appropriate header for basename(3).

Index: imap/relocate_by_id.c
--- imap/relocate_by_id.c.orig
+++ imap/relocate_by_id.c
@@ -42,6 +42,7 @@
 
 #include <config.h>
 
+#include <libgen.h>
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif