Reinstate "regcomp.c: Make C-array inversion lists const"

Reinstate "regcomp.c: Make C-array inversion lists const"

This reverts commit 18505f093a44607b687ae5fe644872f835f66313, which
reverted 241136e0ed70738cccd6c4b20ce12b26231f30e5, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

The inversion lists that are compiled into a C header are now const.