jitterentropy: use wipememory for jent_memset_secure

* random/jitterentropy-base-user.h (jent_memset_secure): Use wipememory.

Patch removes GCC inline assembly dependency on compiler barrier by
instead using wipememory for clearing memory.

• GnuPG-bug-id: T5523
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>