vtest: fix -Wstrict-prototypes build failure

Fixes: 7f644a81 ("Revert "meson: tighten up compile warnings with -Werror=all"")
Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Part-of: <https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/1165>
1 file changed