test/dbus-device-reservation: fix include path

The extra `../` only works if the build directory is an immediate child of the
source directory. In all the other configurations this breaks the build.

Dropping the extra `../` makes use of the `-I${SOURCE_DIR}` provided by meson.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>

tests/modules/dbus-device-reservation.c

@@ -9,7 +9,7 @@
 #include <wp/wp.h>
 #include <pipewire/pipewire.h>
 
-#include "../../../modules/module-monitor/dbus-device-reservation.h"
+#include "../../modules/module-monitor/dbus-device-reservation.h"
 
 typedef struct {
   GTestDBus *dbus_test;