Fix compile warnings printing size_t

[awiddersheim]

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 45b7590.

This seems like the best fix but perhaps there is something better.