jtag: make cmd_queue_scan_field_clone public

jtag: make cmd_queue_scan_field_clone public

This patch makes the cmd_queue_scan_field_clone function public. This
permits targets to insert fields without affecting the submitted
scan_field list. This will be used in an upcoming target implementation
that needs to insert additional padding bits.

Change-Id: I8fbd3b9b4e413432471f4f1444048932c8fa189e
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/4082
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>