veriexec: Correctly export symbols

There's no symbol named 'mac_veriexec_get_executable_flags', the right
one should be the function 'mac_veriexec_metadata_get_executable_flags()'.

Reviewed by: stevek
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42133

(cherry picked from commit f34c9c4e3bdc2b8bffae4ac26897e0e847e9f76f)
(cherry picked from commit d8aaf09792338fae07b9618665ea9612b7b92a6e)