www/bookmarkbridge: Fix build with Clang 6

xbelwrite.cpp:90:7: error: declaration of variable 'QString' with deduced type 'auto' requires an initializer

auto    QString tempstr;
        ^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/bookmarkbridge-0.76_6.log

• Deprecate it. It is doubtful this works correctly with modern browsers anymore.