gallium: Make pipe_query_result::batch array length non-zero.

Zero length arrays are non standard:

   warning C4200: nonstandard extension used : zero-sized array in struct/union
   Cannot generate copy-ctor or copy-assignment operator when UDT contains a zero-sized array

And all code does `N * sizeof query_result->batch[0]`, so it should work
exactly the same.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/include/pipe/p_defines.h

@@ -822,8 +822,8 @@ union pipe_query_result
    /* PIPE_QUERY_PIPELINE_STATISTICS */
    struct pipe_query_data_pipeline_statistics pipeline_statistics;
 
-   /* batch queries */
-   union pipe_numeric_type_union batch[0];
+   /* batch queries (variable length) */
+   union pipe_numeric_type_union batch[1];
 };
 
 union pipe_color_union