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.