xwayland: libxshmfence is needed when dri3 is enabled

* The build error happens already at configure time:

| meson.build: ERROR: Problem encountered: DRI3 requested, but xshmfence not found

Signed-off-by: Carlos Alberto Lopez Perez <clopez@igalia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>