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 186acfb ("tests/qtest: Depend on dbus_display1_dep") no
longer relevant, as dbus-display1.c will be recompiled.

Fixes: c172136 ("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>
(cherry picked from commit d4069a8)

ui/meson.build

@@ -91,8 +91,7 @@ if dbus_display
                                           '--interface-prefix', 'org.qemu.',
                                           '--c-namespace', 'QemuDBus',
                                           '--generate-c-code', '@BASENAME@'])
-  dbus_display1_lib = static_library('dbus-display1', dbus_display1, dependencies: gio)
-  dbus_display1_dep = declare_dependency(link_with: dbus_display1_lib, sources: dbus_display1[0])
+  dbus_display1_dep = declare_dependency(sources: dbus_display1, dependencies: gio)
   dbus_ss.add(when: [gio, dbus_display1_dep],
               if_true: [files(
                 'dbus-chardev.c',