devtools: Fix verneed section parsing in pixie

I misunderstood the ELF specification for version symbols (verneed):
The `vn_aux` pointer is relative to the main verneed record, not the
start of the section.

This caused many symbols to not be versioned properly in the return
value of `elf.dyn_symbols`. This was discovered in bitcoin#21454.

Fix it by correcting the offset computation.

contrib/devtools/pixie.py

@@ -217,7 +217,7 @@ def _parse_verneed(section: Section, strings: bytes, eh: ELFHeader) -> Dict[int,
     result = {}
     while True:
         verneed = Verneed(data, ofs, eh)
-        aofs = verneed.vn_aux
+        aofs = ofs + verneed.vn_aux
         while True:
             vernaux = Vernaux(data, aofs, eh, strings)
             result[vernaux.vna_other] = vernaux.name