package/smack: fix musl build

The build error was not yet found by the autobuilders:

common.c: In function ‘clear’:
common.c:36:12: error: ‘PATH_MAX’ undeclared (first use in this function)
  char path[PATH_MAX];
            ^

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>