util: add an ARRAY_SIZE helper.

Bug: None
Test: `make tests` pass
Change-Id: Iecd2015b7ff96cdd78ff61c905ad065e272c2885
diff --git a/util.h b/util.h
index d63590e..f019f00 100644
--- a/util.h
+++ b/util.h
@@ -35,6 +35,8 @@
 #define info(_msg, ...) \
 	syslog(LOG_INFO, "libminijail[%d]: " _msg, getpid(), ## __VA_ARGS__)
 
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+
 extern const char *log_syscalls[];
 extern const size_t log_syscalls_len;