block/iscsi: Restrict Linux-specific code

block/iscsi: Restrict Linux-specific code

Some Linux specific code is missing guards, leading to
build failure on OSX:

  $ sudo brew install libiscsi
  $ ./configure && make
  [...]
    CC      block/iscsi.o
  qemu/block/iscsi.c:338:24: error: 'iscsi_aiocb_info' defined but not used [-Werror=unused-const-variable=]
   static const AIOCBInfo iscsi_aiocb_info = {
                          ^~~~~~~~~~~~~~~~
  qemu/block/iscsi.c:168:1: error: 'iscsi_schedule_bh' defined but not used [-Werror=unused-function]
   iscsi_schedule_bh(IscsiAIOCB *acb)
   ^~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Add guards to restrict this code for Linux.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190220000553.28438-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>