Revert "flex: Version bump 2.5.39"

The flex bump breaks at least two packages:

 - host-thrift, see http://autobuild.buildroot.org/results/759/7590122fd85b644ae0886a127005188d1f882bef/
 - ipsec-tools, see http://autobuild.buildroot.org/results/238/238296556663d52015beb43df97106f6e164ce55/



No simple fix was found for now, so reverting is the easiest solution
until some time is found to look at doing a flex bump that doesn't
break those packages.

This reverts commit 931d9fba.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>