autoPatchelfHook: Silence errors in isExecutable

The "maxx" package recursively runs isExecutable on a bunch of files and
since the change to use "readelf" instead of "file" a lot of errors like
this one are printed during build:

  readelf: Error: Not an ELF file - it has the wrong magic bytes at the
  start

While the isExecutable was never meant to be used outside of the
autoPatchelfHook, it's still a good idea to silence the errors because
whenever readelf fails, it clearly indicates that the file in question
is not a valid ELF file.

Signed-off-by: aszlig <aszlig@nix.build>