alsactl: fix compilation when building in a subdir

alsactl: fix compilation when building in a subdir

Fixes: 613372d
Fixes: cff2d1c
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"
      |          ^~~~~~~~~~~~~

Fixes: https://github.com/alsa-project/alsa-utils/pull/213
Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>