Add missing include

Index: mpg123ctl.h
--- mpg123ctl.h.orig
+++ mpg123ctl.h
@@ -14,6 +14,7 @@
 
 #define __MPG123CTL__
 
+#include <ctype.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <signal.h>
