Add OSSL_NELEM macro.

Add OSSL_NELEM macro to e_os.h to determine the number of elements in an
array.

Reviewed-by: Tim Hudson <tjh@openssl.org>
diff --git a/e_os.h b/e_os.h
index f4a427a..7f94bfc 100644
--- a/e_os.h
+++ b/e_os.h
@@ -689,6 +689,8 @@
 #  endif
 # endif
 
+#define OSSL_NELEM(x)    (sizeof(x)/sizeof(x[0]))
+
 #ifdef  __cplusplus
 }
 #endif