Suppress build warning for oniguruma

[kuba--]

Every time when we build mysql (and/or gitbase) with oniguruma you'll get an warning:

# github.com/src-d/gitbase/vendor/github.com/moovweb/rubex
chelper.c:161:43: warning: passing 'const OnigUChar *' (aka 'const unsigned char *') to parameter of type 'const char *' converts between pointers to integer types with different sign [-Wpointer-sign]
/usr/include/secure/_string.h:124:34: note: expanded from macro 'strncpy'

Because go binding for oniguruma is archived, we need to fork it to src-d/go-oniguruma first.
Keep in mind that only go1 branch is useful for us.

[kuba--]

it was done here: https://github.com/src-d/go-oniguruma
So the task now is to adopt a new rubex fork in mysql.