Fix version numbers in intrpvar.h comments

Fix version numbers in intrpvar.h comments

There are two version numbers in intrpvar.h that have been repeatedly but
confusingly bumped by an older version of Porting/bump-perl-version. Now
that the porting script ignores intrpvar.h, it's better to restore the
version numbers to the way they were when they were originally added. The
comment for PERL_LAST_5_18_0_INTERP_MEMBER was introduced in commit
d399cf59bde32e412ae99791ae46a871c7337b42, and the comments for PL_timesbuf
was introduced in 25983af42cdcf2dc1fea6717dac7aac441b6301d.