Add missing include
https://bugs.gentoo.org/878641
--- a/src/main.c
+++ b/src/main.c
@@ -25,6 +25,7 @@
 #include "config.h"
 #endif
 
+#include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>