ui: compile dbus-display1.c with -fPIC as necessary

ui: compile dbus-display1.c with -fPIC as necessary

Building dbus-display1.c explicitly as a static library drops -fPIC by
default, which may not be correct if it ends up linked to a shared
library.

Let the target decide how to build the unit, with or without -fPIC. This
makes commit 186acfbaf7 ("tests/qtest: Depend on dbus_display1_dep") no
longer relevant, as dbus-display1.c will be recompiled.

Fixes: c172136ea33 ("meson: ensure dbus-display generated code is built
before other units")

Reported-by: Olaf Hering <olaf@aepfle.de>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>