[Vulkan] Port ac6_ground_fix vertex-fetch nudge to SPIR-V

src/xenia/gpu/gpu_flags.h

@@ -42,6 +42,8 @@ DECLARE_bool(async_shader_compilation);
 
 DECLARE_bool(gpu_3d_to_2d_texture);
 
+DECLARE_bool(ac6_ground_fix);
+
 #define XE_GPU_FINE_GRAINED_DRAW_SCOPES 1
 
 #endif  // XENIA_GPU_GPU_FLAGS_H_