[gdb/build] Fix buildbreaker on x86_64-freebsd in gdb/fbsd-nat.c

Following commit 1ad8737b3c5 ("gdb: change inf_threads_iterator to yield
references"), we're running into a build breaker on x86_64-freebsd, in
gdb/fbsd-nat.c.

Fix this.

Tested by completing a build on x86_64-freebsd and running the TUI test-cases.