alter: fix potential UB in RebuildFuncIndex

Method `func_index_def_new` is used to calculate an argument for base
class constructor when it is not built yet. Fortunately, it does not
use class fields - let's make it static to use it before class
initialization legitimately.

Part of tarantool/security#96

NO_TEST=no behaviour changes
NO_CHANGELOG=no behaviour changes
NO_DOC=no behaviour changes

(cherry picked from commit b5163ef7)