The ssize_t type requires the unistd.h header. This fixes build with musl
libc:
In file included from ../include/tinyalsa/limits.h:32:0,
from limits.c:1:
../include/tinyalsa/interval.h:38:2: error: unknown type name 'ssize_t'
ssize_t max;
^
Signed-off-by: Baruch Siach <baruch@tkos.co.il>