drm/amd/display: add virtual_setup_stream_attribute decl to header

Smatch reports this issue
virtual_link_hwss.c:32:6: warning: symbol
  'virtual_setup_stream_attribute' was not declared.
  Should it be static?

virtual_setup_stream_attribute is only used in
virtual_link_hwss.c, but the other functions in the
file are declared in the header file and used elsewhere.
For consistency, add the virtual_setup_stream_attribute
decl to virtual_link_hwss.h.
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/virtual/virtual_link_hwss.h

@@ -28,6 +28,7 @@
 #include "core_types.h"
 
 void virtual_setup_stream_encoder(struct pipe_ctx *pipe_ctx);
+void virtual_setup_stream_attribute(struct pipe_ctx *pipe_ctx);
 void virtual_reset_stream_encoder(struct pipe_ctx *pipe_ctx);
 const struct link_hwss *get_virtual_link_hwss(void);