o_str.c: Windows doesn't have <strings.h>, and since we use _strnicmp() and

_stricmp() on that platform, use the appropriate header file for it,
<string.h>.
o_str.h: we only want to get size_t, which is defined in <stddef.h>.

Philippe Bougeret <philippe.bougeret@freesbee.fr> notified us about Windows
not having a <strings.h>