selftests/powerpc/ptrace: Set LOCAL_HDRS

Set LOCAL_HDRS so header changes cause rebuilds. The lib.mk logic adds
all the headers in LOCAL_HDRS as dependencies, so there's no need to
also list them explicitly.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-2-mpe@ellerman.id.au