socketcand: resolve GCC5 related issue

This patch removes inline keyword from header and source
files.

Fixes following autobuild failure:
http://autobuild.buildroot.net/results/8ea/8eac9832554dbd1f2e14cf54e5c99e6bf4dfc2cc/



Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>