Merge pull request #599 from h2o/kazuho/alpine

add missing include (otherwise build fails on alpine 3.21)

src/cli.c

@@ -33,6 +33,7 @@
 #include <netinet/udp.h>
 #include <fcntl.h>
 #include <netdb.h>
+#include <signal.h>
 #include <stdio.h>
 #include <unistd.h>
 #include <picotls.h>