slang: forcibly disable oniguruma support

If the host has oniguruma installed then slang can detect it and try to
(wrongly) link against it resulting in build failure. Fixes:
http://autobuild.buildroot.net/results/51f/51ff079ebbd363e0445d226ec80bc28e524fd88c/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>