4.8 Kernel Compiling Error

mkdep -f
.depend -a   -nostdinc -D_KERNEL -DKLD_MODULE -I- -I. -I_at_ -I_at_/../include -I/
usr/obj/usr/src/i386/usr/include
/usr/src/sys/modules/iir/../../dev/iir/iir.c
/usr/src/sys/modules/iir/../../dev/iir/iir_ctrl.c
/usr/src/sys/modules/iir/../../dev/iir/iir_pci.c
/usr/src/sys/modules/iir/../../dev/iir/iir.c:56: stddef.h: No such file or
directory
mkdep: compile failed
*** Error code 1

Stop in /usr/src/sys/modules/iir.
*** Error code 1

Stop in /usr/src/sys/modules.
*** Error code 1

Stop in /usr/obj/usr/src/sys/MYKERNEL.
*** Error code 1

Stop in /usr/src.
*** Error code 1

Stop in /usr/src.

I have used src from the cdrom and from FTP1, regardless I am unable to
compile the kernel.

Any assistance would be appreciated

Thank you.