eBPF files are being included in user emulators, which is useless and
also breaks compilation because ebpf/trace-events is only processed
if a system emulator is included in the build.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/566


Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>