FDPIC: Fix memory leak

The shdr4extnum variable isn't being freed in the cleanup process of
elf_fdpic_core_dump().

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>