Fix compile warnings printing size_t

Every time I was building on Windows I was getting the following warnings:

```
os_crypto/shared/msgs.c:555:21: warning: format '%lu' expects type 'long unsigned int', but argument 4 has type 'size_t'
os_crypto/shared/msgs.c:555:21: warning: format '%lu' expects type 'long unsigned int', but argument 5 has type 'size_t'
os_crypto/shared/msgs.c:555:21: warning: format '%lu' expects type 'long unsigned int', but argument 6 has type 'size_t'
```
I think these were coming from the changes made in https://github.com/ossec/ossec-hids/commit/45b75900deb3eec252611697dab18c7ae1790b19. 

This seems like the best fix but perhaps there is something better.