I've run into this as well, using HP's C/iX compiler.  I created my own
prototype, as follows:
#ifdef __mpexl
   /* for some reason, MPE/iX doesn't declare this  */
   /* anywhere, so we will                          */
   char* inet_ntoa(struct in_addr in);
#endif
Works fine.  My program is linking with SOCKETRL.NET.SYS,
LIBCPX.LIB.SYS and LIBMANSI.LIB.SYS.  Forget which of these contains the
actual code.
Hope this helps, Bob