libc: Add pdfork to the list of interposed system calls

Otherwise the asm stub is used and libthr interposition does not work.

Reviewed by: kib
Fixes: 21f749da82e7 ("libthr: wrap pdfork(2), same as fork(2).")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit cbdec8db18b533f6d7be329da151d6b61df3f98b)