Fix "Unescaped left brace in regex is deprecated here" warnings from Perl 5.26.
Reported and patch by Jehan <jehan@girinstud.io> <jehan@zemarmot.net> at
<https://savannah.gnu.org/bugs/?54088>.
* gettext-tools/doc/texi2html: Escape braces in regular expressions.
* gettext-runtime/libasprintf/texi2html: Likewise.
