exterr: Sort output from make_libc_exterr_cat_filenames.sh

Otherwise the script may permute the order of entries in the file since
find(1) output is not stable.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54669

(cherry picked from commit bda6ed2ee4d8e836b1b2f8ca7a6ed72034e5f231)