fix nob_nprocs build on FreeBSD/BSD under -std=c99

[SpartanSf]

to reproduce issue fixed here:

• cc -std=c99 -O2 -Wall -Wextra -o nob nob.c on FreeBSD (i specifically tested 14.5, applies almost anywhere though)

root cause:

• non-POSIX sysconf name hidden

fix:

• guard + hw.ncpu fallback, tested (Linux gcc/clang clean, works on BSD)