Fix a compiler warning on OpenBSD in test-memwipe.c

In test-memwipe.c, malloc_options needs to be declared extern:

src/test/test-memwipe.c:50:13: warning: no previous extern declaration for non-static variable 'malloc_options' [-Wmissing-variable-declarations]
const char *malloc_options="sufjj";

(malloc_options is only used on OpenBSD)

PR to follow.

Trac:
Username: kjak

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