ivi-shell: make 'extern' the definition of ivi_layout_interface in H file

	the lack of 'extern' keyword at the definition of
	ivi_layout_interface in the H file resulted a local,
	empty duplicate of the struct whenever the H file was
	included. This duplicate shadowed the original function
	table and since it was all empty it resulted crashes whenever
	someone tried to use it.

	Modules that access the same table via dlopen/dlsym functioned
	properly however.