alsactl: fix compilation when building in a subdir

alsa-project/alsa-utils pull request #213 was opened from heitbaum:

Fixes: 613372d Fixes: cff2d1c Fixes: #212

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" | ^~~~~~~~~~~~~ ```

Request URL : https://github.com/alsa-project/alsa-utils/pull/213 Patch URL : https://github.com/alsa-project/alsa-utils/pull/213.patch Repository URL: https://github.com/alsa-project/alsa-utils