util/mkdef.pl: handle line terminators correctly

When parsing the header files, mkdef.pl didn't clear the line
terminator properly.  In most cases, this didn't matter, but there
were moments when this caused parsing errors (such as CRLFs in certain
cases).

Fixes #4267

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4304)