veristat: Strerror expects positive number (errno)

Before:

  ./veristat -G @foobar iters.bpf.o
  Failed to open presets in 'foobar': Unknown error -2
  ...

After:

  ./veristat -G @foobar iters.bpf.o
  Failed to open presets in 'foobar': No such file or directory
  ...

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>
Link: https://lore.kernel.org/bpf/20250301000147.1583999-3-eddyz87@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>