samples/bpf: Pass TPROGS_USER_CFLAGS to libbpf makefile

Before commit [1], the value of a variable TPROGS_USER_CFLAGS was
passed to libbpf make command as a part of EXTRA_CFLAGS.
This commit makes sure that the value of TPROGS_USER_CFLAGS is still
passed to libbpf make command, in order to maintain backwards build
scripts compatibility.

[1] commit 5a6ea702 ("samples/bpf: Remove unnecessary -I flags from libbpf EXTRA_CFLAGS")

Fixes: 5a6ea702 ("samples/bpf: Remove unnecessary -I flags from libbpf EXTRA_CFLAGS")
Suggested-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Viktor Malik <vmalik@redhat.com>
Link: https://lore.kernel.org/bpf/20241204173416.142240-1-eddyz87@gmail.com