contrib/plugins: add compat for g_memdup2

contrib/plugins: add compat for g_memdup2

We were premature if bumping this because some of our builds are still
on older glibs. Just copy the compat handler for now and we can remove
it later.

Fixes: ee293103b0 (plugins: update lockstep to use g_memdup2)
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2161
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-14-alex.bennee@linaro.org>
(cherry picked from commit 44e794896759236885f6d30d1f6b9b8b76355d52)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2681 in 8.2.x