Ak79,
On my Linux machine, the netdb.h file is under the same directory as all the other regular include files for compiling MPC8313E targeted linux apps.
Here is where I found it on my machine:
alan@raptor:/opt/mtwk/usr/local/powerpc-linux/gcc-3.4.3-glibc-2.3.3/powerpc-linux/include$
ls net*.hnetdb.h
I also know that busybox uses netdb.h, so I'd assume it would be a standard install when LTIB is installed, but as I've recompiled busybox to add some options that were not in the default build of the MPC8313E Linux BSP, I guess there is a chance that the .h file is installed if you build busybox from the ./ltib --configure menu.
Hope this helps.
Cheers,
abartky