malloc: obscure calloc use in tst-calloc

malloc: obscure calloc use in tst-calloc

Similar to a9944a52c967ce76a5894c30d0274b824df43c7a and
f9493a15ea9cfb63a815c00c23142369ec09d8ce, we need to hide calloc use from
the compiler to accommodate GCC's r15-6566-g804e9d55d9e54c change.

First, include tst-malloc-aux.h, but then use `volatile` variables
for size.

The test passes without the tst-malloc-aux.h change but IMO we want
it there for consistency and to avoid future problems (possibly silent).

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>