package/squashfs: fix build on hosts with glibc >= 2.28

Since glibc 2.28, sys/sysmacros.h is no longer included from sys/types.h
This patch fixes the resulting build error by explicitly including
sys/sysmacros.h

See also https://github.com/plougher/squashfs-tools/pull/52



Signed-off-by: Richard Kunze <richard.kunze@web.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>