tools/gdbsx: fix -Wstringop-truncation warning

tools/gdbsx: fix -Wstringop-truncation warning

gcc-8 complains:

    gx_main.c: In function 'prepare_stop_reply':
    gx_main.c:385:9: error: 'strncpy' output truncated before terminating nul copying 6 bytes from a string of the same length [-Werror=stringop-truncation]
             strncpy(buf, "watch:", 6);
             ^~~~~~~~~~~~~~~~~~~~~~~~~

Since terminating '\0' isn't needed here at all, switch to memcpy.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-Acked-by: Juergen Gross <jgross@suse.com>