selftests/seccomp: powerpc: Fix typo in macro variable name

A typo sneaked into the powerpc selftest. Fix the name so it builds again.

Fixes: 46138329

 ("selftests/seccomp: powerpc: Fix seccomp return value testing")
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/lkml/87y2ix2895.fsf@mpe.ellerman.id.au
Signed-off-by: Kees Cook <keescook@chromium.org>