linker: Silence GCC unused parameter warnings

linker: Silence GCC unused parameter warnings

linker.cpp:320:55: warning: unused parameter 'ir' [-Wunused-parameter]
    virtual ir_visitor_status visit_leave(ir_function *ir)
                                                       ^
linker.cpp:327:53: warning: unused parameter 'ir' [-Wunused-parameter]
    virtual ir_visitor_status visit_leave(ir_return *ir)
                                                     ^
linker.cpp:333:49: warning: unused parameter 'ir' [-Wunused-parameter]
    virtual ir_visitor_status visit_enter(ir_if *ir)
                                                 ^
linker.cpp:339:49: warning: unused parameter 'ir' [-Wunused-parameter]
    virtual ir_visitor_status visit_leave(ir_if *ir)
                                                 ^
linker.cpp:345:51: warning: unused parameter 'ir' [-Wunused-parameter]
    virtual ir_visitor_status visit_enter(ir_loop *ir)
                                                   ^
linker.cpp:351:51: warning: unused parameter 'ir' [-Wunused-parameter]
    virtual ir_visitor_status visit_leave(ir_loop *ir)
                                                   ^
linker.cpp:2824:53: warning: unused parameter 'ctx' [-Wunused-parameter]
 link_calculate_subroutine_compat(struct gl_context *ctx, struct gl_shader_program *prog)
                                                     ^
linker.cpp:2854:47: warning: unused parameter 'ctx' [-Wunused-parameter]
 check_subroutine_resources(struct gl_context *ctx, struct gl_shader_program *prog)
                                               ^
linker.cpp:3368:49: warning: unused parameter 'ctx' [-Wunused-parameter]
 link_assign_subroutine_types(struct gl_context *ctx,
                                                 ^

Also make link_assign_subroutine_types static since it is only called
from this file.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>