vtun: fix gcc5 build

Add -std=gnu89 to CFLAGS to keep pre gcc5 inline function symbols visibility
rules.

Fixes:
http://autobuild.buildroot.net/results/a7e/a7e684174b90a852318a7a8bf41c4f477d159d56/
http://autobuild.buildroot.net/results/ab0/ab01f8e0467e85322c63b81bb7e84fd251e5d556/
http://autobuild.buildroot.net/results/117/117a68bf648b050dabed8088d1165c1d7d9301f0/



and more.

[Peter: add a comment explaining why]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>