squashfs3: fix build with uClibc

The squashfs3 package uses the old get_nprocs() GNU extension which does not
exist in uClibc. This has already been fixed in newer squashfs releases
(>=4.0). The patch is similar to the one committed in
dfa2a513

[Peter: add patch header]
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>