Changes

build: make linux/if_packet.h usage optional

Check for presence of linux/if_packet.h in configure and
guard its usage with HAVE_LINUX_IF_PACKET_H.

This avoids build failures on non-Linux platforms or
environments where the header is unavailable.

Change-Id: I8a02b0676e65fa4b6191073c84e0646a2e67a010

build: remove unused sys/timerfd.h include

The header is not used in scheduler_trx.c and breaks builds on
platforms where timerfd is not available (e.g. Web/emscripten).

Remove the unused include.

Change-Id: Id5a700c865f984e8f99199a1e365bb7f1462030a