flite: fix <pkg>_LICENSE_FILES

Following commit dd4bcebd ("flite: use
FLITE_STRIP_COMPONENTS and FLITE_SUBDIR"), the FLITE_LICENSE_FILES was
no longer pointing to existing directories, which was breaking
legal-info.

Fixes:

  http://autobuild.buildroot.org/results/798/7983ec41a3f136bd997ebbfc110c7fc51ad79717/



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