gdbserver: fix build on NetBSD The function remove_thread() was changed to a method in 2500e7d7d (gdbserver: make remove_thread a method of process_info). Signed-off-by: Wataru Ashihara <wsh@iij.ad.jp> Change-Id: I4b2d8a6f84b5329b8d450b268fa9453fe424914e