lib/user_busy.c: Include <utmpx.h>

Since:
- utmpx APIs are used in non-Linux code blocks
- <utmpx.h> is already unconditionally included in Linux parts in other
  files
then unconditionally include it in this file as well.

Signed-off-by: Pino Toscano <toscano.pino@tiscali.it>

lib/user_busy.c

@@ -17,6 +17,7 @@
 #include <dirent.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <utmpx.h>
 
 #include "atoi/getnum.h"
 #include "defines.h"