Move declarations of debug_threads and using_threads to header
Instead of declaring debug_threads and using_threads in several .c files, declare them in debug.h. gdb/gdbserver/ChangeLog: 2019-09-20 Christian Biesinger <email@example.com> * debug.c (debug_threads): Remove comment in favor of the header. * debug.h (using_threads): Add declaration. (debug_threads): Add comment. * linux-aarch64-low.c: Include debug.h and remove declaration of debug_threads. * nto-low.c: Likewise. * remote-utils.c: Likewise. * thread-db.c: Likewise.