src/magnifier: fix compile on ancient GCC versions

src/magnifier: fix compile on ancient GCC versions

Fixes
```
../src/magnifier.c:153:3: error: missing initializer for field ‘clip’ of ‘struct wlr_render_rect_options’ [-Werror=missing-field-initializers]
  153 |   };
      |   ^
In file included from ../subprojects/wlroots/include/wlr/render/wlr_renderer.h:14,
                 from ../subprojects/wlroots/include/wlr/types/wlr_output.h:17,
                 from ../src/magnifier.c:5:
../subprojects/wlroots/include/wlr/render/pass.h:115:27: note: ‘clip’ declared here
  115 |  const pixman_region32_t *clip
```