ui: fix path to dbus-display1.h

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>
diff --git a/ui/dbus.h b/ui/dbus.h
index c001c11..9c149e7 100644
--- a/ui/dbus.h
+++ b/ui/dbus.h
@@ -31,7 +31,7 @@
 #include "ui/console.h"
 #include "ui/clipboard.h"
 
-#include "dbus-display1.h"
+#include "ui/dbus-display1.h"
 
 typedef struct DBusClipboardRequest {
     GDBusMethodInvocation *invocation;