media-sound/teamspeak-client: add workaround for Qt 5.12

media-sound/teamspeak-client: add workaround for Qt 5.12

QtWebEngine versions before 5.12 used to reexport a few symbols like
calloc with version Qt_5 that were exported by mistake once. These are
no longer exported in 5.12 (probably a Qt bug), and teamspeak-client
does not officially support it (only the bundled QtWebEngine 5.6.1 is
supported).

Add a `overrides.so` shared library that reexports them by taking this
code from Qt under (L)GPL and preload it using LD_PRELOAD to make the
symbols available.

Closes: https://bugs.gentoo.org/675874
Signed-off-by: Denis Lisov <dennis.lissov@gmail.com>
Signed-off-by: Michał Górny <mgorny@gentoo.org>