While the source directory is always included in the include path,
the corresponding directory in the build tree is not.  Therefore,
custom_targets (e.g. ui/dbus-display1.h) must be referred to using
the full path.

This avoids a build failure when ui/dbus-chardev.c is not built as
a module:

In file included from ../ui/dbus-chardev.c:32:
../ui/dbus.h:34:10: fatal error: dbus-display1.h: No such file or directory
   34 | #include "dbus-display1.h"
      |          ^~~~~~~~~~~~~~~~~

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>