ebpf: really include it only in system emulators

ebpf: really include it only in system emulators

eBPF libraries are being included in user emulators, which is useless and
also breaks --static compilation if a shared library for libbpf is
present in the system.

Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211012162252.263933-1-pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-bt: Warner Losh <imp@bsdimp.com>
Message-Id: <20211026102234.3961636-13-alex.bennee@linaro.org>