ui: dbus-display requires CONFIG_GBM

ui: dbus-display requires CONFIG_GBM

Without CONFIG_GBM, compiling dbus-display fails with

../ui/dbus.c: In function ‘dbus_create_context’:
../ui/dbus.c:47:20: error: ‘qemu_egl_rn_ctx’ undeclared (first use in this function); did you mean ‘qemu_egl_init_ctx’?
   47 |                    qemu_egl_rn_ctx);
      |                    ^~~~~~~~~~~~~~~
      |                    qemu_egl_init_ctx
../ui/dbus.c:47:20: note: each undeclared identifier is reported only once for each function it appears in

and many other similar errors, because include/ui/egl-helpers.h only has
these declaration if gbm is found on the system.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1108
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>