flex: propagate m4 dependency on wchar

Commit fe6a9e5e missed the fact that
m4 has a dependency on wide char support, so this dependency should be
propagated to flex.

Fixes:

  http://autobuild.buildroot.org/results/495/49502b2e33a346b2fcebf5e2da00af2661b54d6e/build-end.log



[Peter: Note where wchar dep comes from, show comment if not available]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>