The HZ define is not exposed by musl libc.

--- a/src/os/linux/linux_sigar.c
+++ b/src/os/linux/linux_sigar.c
@@ -23,6 +23,7 @@
 #include <sys/stat.h>
 #include <sys/times.h>
 #include <sys/utsname.h>
+#include <linux/param.h>
 
 #include "sigar.h"
 #include "sigar_private.h"