autoPatchelfHook: Skip on missing segment headers

If the file in question is not a shared object file but an ELF, we
really want to skip the file, because we won't have anything to patch
there.

For example if the file is created via "gcc -c -o foo.o foo.c", we don't
get a segment header and so far autoPatchelf was trying to patch such a
file.

By checking for missing segment headers, we're now no longer going to
attempt patching such a file.

Signed-off-by: aszlig <aszlig@nix.build>
Reported-by: Sander van der Burg <svanderburg@gmail.com>