elf: Compile with -Wextra

-Wextra enables a bunch of rather useful checks which this fixes.

This changes the return value for the case when no ELF headers were found
to avoid (ugly-ish) cast of -1 to unsigned.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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Changes:
v2:
* cast the return value to (int)
* added missing (long)