softpipe: Add a dummy field to sp_fragment_shader_variant_key

MSVC doesn't support 0-size structs in C.

Fixes: 0b7a0d1a ("softpipe: Use the draw module's poly stipple handling, like llvmpipe.")
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14312>

src/gallium/drivers/softpipe/sp_state.h

@@ -64,6 +64,8 @@ struct vertex_info;
 
 struct sp_fragment_shader_variant_key
 {
+   /* Standard C doesn't allow 0-size structs */
+   int dummy;
 };