zink: use correct define value for reserved slot count in ntv

this is zero-indexed, so we need to include the zero index in the count

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5592>

src/gallium/drivers/zink/nir_to_spirv/nir_to_spirv.c

@@ -87,7 +87,7 @@ static unsigned slot_pack_map[] = {
    [VARYING_SLOT_VIEW_INDEX] = 9, /* input/output */
    [VARYING_SLOT_VIEWPORT_MASK] = 10, /* output only */
 };
-#define NTV_MIN_RESERVED_SLOTS 10
+#define NTV_MIN_RESERVED_SLOTS 11
 
 struct ntv_context {
    struct spirv_builder builder;