doc: add missing struct keyword (thanks Stojce and Emmanuel for the BR)

doc/doxygen/chapters/07data_management.doxy

@@ -120,7 +120,7 @@ starpu_vector_data_register(&handle, STARPU_MAIN_RAM, (uintptr_t)vector,
                             NX, sizeof(vector[0]));
 
 /* Partition the vector in PARTS sub-vectors */
-starpu_data_filter f =
+struct starpu_data_filter f =
 {
     .filter_func = starpu_vector_filter_block,
     .nchildren = PARTS