FIX tools: Fix fix_client link error

Reported by Marat:

  tools/fix/fix_client.o: In function `fix_client_order':
  /home/maratrus/libtrading/tools/fix/fix_client.c:317: undefined reference to `fmin'
  /home/maratrus/libtrading/tools/fix/fix_client.c:318: undefined reference to `fmax'
  collect2: ld returned 1 exit status

Signed-off-by: Pekka Enberg <penberg@iki.fi>