Surfaces and panners should not export headers

Those are not shared libraries but only dynamically loaded
plugin objects. Their headers are their own.

libs/surfaces/websockets/wscript

@@ -31,7 +31,6 @@ def build(bld):
             resources.cc
             json.cc
     '''
-    obj.export_includes = ['.']
     obj.defines      = [ 'PACKAGE="ardour_websockets"' ]
     obj.defines     += [ 'ARDOURSURFACE_DLL_EXPORTS' ]
     obj.includes     = ['.', './websockets']