gdb: long running "monitor mww" now works w/gdb

invoke keep_alive() to make sure that the default 2000ms
timeout does not trigger.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

src/target/target.c

@@ -2355,6 +2355,8 @@ static int target_fill_mem(struct target *target,
 		{
 			break;
 		}
+		/* avoid GDB timeouts */
+		keep_alive();
 	}
 	free(target_buf);