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.

(cherry picked from commit 7f601f7)
Cherry-pick-repository: https://xenbits.xen.org/gitweb/?p=xen.git
Issue: ALB-3594
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

tools/debugger/gdbsx/gx/gx_main.c

@@ -382,7 +382,7 @@ prepare_stop_reply(enum target_signal sig, char *buf, vcpuid_t vcpu)
 
     /* TBD: check if we stopped because of watchpoint */
     if (watchpoint_stop()) {
-        strncpy(buf, "watch:", 6);
+        memcpy(buf, "watch:", 6);
         buf += 6;
         /* TBD: **/
     }