NetworkPkg/NetworkPkg.dsc: Add the instance of library class [SafeIntLib].

NetworkPkg/NetworkPkg.dsc: Add the instance of library class [SafeIntLib].

This patch is to add the instance of library class [SafeIntLib] to fix the
NetworkPkg build error, which is caused by the commit of 2167c7f7 that the
TlsLib will always consume SafeIntLib.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Long Qin <qin.long@intel.com>
Cc: Bi Dandan <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Long Qin <qin.long@intel.com>