Fix compilation on Mac OS

> src/tarantool/src/box/vinyl.c:2111:33: error: initializer element is not a compile-time constant
>         static const double weight = 1 - exp(-VY_QUOTA_UPDATE_INTERVAL /
>                                      ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Remove the "static" qualifier. It is not really needed as any sane
compiler will pre-calculate the value of 'weight' at compile time
(checked on gcc 6.3.0 with -O0).