manual: don't match nonexistent file while preprocessing

manual: don't match nonexistent file while preprocessing

Authsrv(6) used to be called auth(6) before
the fourth edition and was preprocessed by
eqn(6) before the third edition. The mkfile
was never updated to reflect the changes.

All other files are accounted for:

% cd /sys/man
% for(i in TS EQ PS G1) echo $i `{grep -l '^\.'$i ?/[0-9a-z]*}
TS 1/tbl
EQ 1/eqn 3/usb
PS 1/pic
G1 1/grap
%