squashfs: fix build with gcc 7

gcc 7 with -Os (optimize for size) takes the liberty to remove the code of
inline function entirely. This leads to undefined function references at link
time. Restore gcc original inline behaviour to fix this issue.

Fixes:
http://autobuild.buildroot.net/results/3c5/3c5b1d799dce3ba361d618330c242bf4eba76019/
http://autobuild.buildroot.net/results/09f/09f350b62e2486404b78222dce211400bb233000/
http://autobuild.buildroot.net/results/693/693960ed7c01622c756dcc929e83b3b713c16ccc/



Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>