lib: add array_size() helper

implement array_size as sizeof(array) / sizeof(array element)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/memory.h b/lib/memory.h
index a7eddce..2396223 100644
--- a/lib/memory.h
+++ b/lib/memory.h
@@ -21,6 +21,8 @@
 #ifndef _ZEBRA_MEMORY_H
 #define _ZEBRA_MEMORY_H
 
+#define array_size(ar) (sizeof(ar) / sizeof(ar[0]))
+
 /* For pretty printing of memory allocate information. */
 struct memory_list
 {