Missing timers.h file for mystery1_x.cc

On file `mystery1_x.cc`, there is an include of `timers.h` which is missing.

Trying to compile it:

```bash
g++ -O2 mystery1_x.cc -o mystery1_x
```

produce the following error:

```bash
mystery1_x.cc:7:10: fatal error: timers.h: No such file or directory
    7 | #include "timers.h"
      |          ^~~~~~~~~~
compilation terminated.
```