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
1 file changed