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>

monitor.c

@@ -2570,11 +2570,11 @@ FdsetInfoList *qmp_query_fdsets(Error **errp)
 
 int monitor_fdset_get_fd(int64_t fdset_id, int flags)
 {
+#ifndef _WIN32
     MonFdset *mon_fdset;
     MonFdsetFd *mon_fdset_fd;
     int mon_fd_flags;
 
-#ifndef _WIN32
     QLIST_FOREACH(mon_fdset, &mon_fdsets, next) {
         if (mon_fdset->id != fdset_id) {
             continue;