configure: fix libpmem configuration option

configure: fix libpmem configuration option

For some reason, libpmem option setting was set to work in an opposite
way (--enable-libpmem disabled it and vice versa). Fixing this so
configuration works properly.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Connor Kuehl <ckuehl@redhat.com>
Message-Id: <20210707075144.82717-1-mrezanin@redhat.com>
Fixes: e36e8c70f6 ("configure, meson: convert libpmem detection to meson", 2021-07-06)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>