Build of alsa-utils 1.2.9 fails when building is a subdir

[GitHub issues - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-utils issue #212 was opened from heitbaum:

Compile errors when building in a subdir:
```
alsactl/alsactl.c:33:10: fatal error: os_compat.h: No such file or directory
   33 | #include "os_compat.h"
      |          ^~~~~~~~~~~~~
alsactl/lock.c:34:10: fatal error: os_compat.h: No such file or directory
   34 | #include "os_compat.h"
      |          ^~~~~~~~~~~~~
```

Issue URL     : https://github.com/alsa-project/alsa-utils/issues/212
Repository URL: https://github.com/alsa-project/alsa-utils