llext: elf.h: fix sign of the `r_addend` field of struct elf32_rela

The r_addend field is signed (it seems unused for now).

Fixes commit a9a82d55 ("llext: use elf_rela_t instead of elf_rel_t")

https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.reloc.html



Signed-off-by: Marc Herbert <marc.herbert@intel.com>