fw_cfg: add required header files

If fw_cfg.h is included alone, gcc gives error messages like these:

  error: unknown type name ‘uint32_t’
  error: unknown type name ‘size_t’
  error: unknown type name ‘hwaddr’
  ...
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-id: d63f8bcdbfbec8135b1b57f9247c513a3e25762c.1366945969.git.hutao@cn.fujitsu.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

include/hw/nvram/fw_cfg.h

 #ifndef FW_CFG_H
 #define FW_CFG_H
 
+#ifndef NO_QEMU_PROTOS
+#include <stdint.h>
+#include <stddef.h>
+
+#include "exec/hwaddr.h"
+#endif
+
 #define FW_CFG_SIGNATURE        0x00
 #define FW_CFG_ID               0x01
 #define FW_CFG_UUID             0x02