bisect-runner.pl should avoid ext/Hash/Util/FieldHash being built twice.

bisect-runner.pl should avoid ext/Hash/Util/FieldHash being built twice.

Commit 550428fe486b1888 stopped the invocation of ext/Hash/Util/Makefile.PL
automatically recursing to ext/Hash/Util/FieldHash/Makefile.PL, as make_ext
was also calling it directly. This then resulted in two makes running in
parallel in ext/Hash/Util/FieldHash, which could all end in tears (and
failed builds). Hence we need to apply this fix if it isn't present, to
avoid spurious build failures during bisect runs.