monitor: avoid declaring unused variables

monitor: avoid declaring unused variables

Some variables are only used on !win32, declare
them only when used.

This avoids a warning in mingw32 build:
  CC    i386-softmmu/monitor.o
/src/qemu/monitor.c: In function 'monitor_fdset_get_fd':
/src/qemu/monitor.c:2575: warning: unused variable 'mon_fd_flags'
/src/qemu/monitor.c:2574: warning: unused variable 'mon_fdset_fd'
/src/qemu/monitor.c:2573: warning: unused variable 'mon_fdset'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>