vulkan: fix sw_frame_deps counter

The code used as a basis was the buffer dependency code, where the
counter was incremented after each buffer, but for the sw_frame dep
API, we only support adding individual frames at a time.

libavutil/vulkan.c

@@ -618,6 +618,8 @@ int ff_vk_exec_add_dep_sw_frame(FFVulkanContext *s, FFVkExecContext *e,
         return AVERROR(ENOMEM);
     }
 
+    e->nb_sw_frame_deps++;
+
     return 0;
 }