MdeModulePkg DxeIpl: Fix IA32 build failure with GCC 5.1.1

MdeModulePkg DxeIpl: Fix IA32 build failure with GCC 5.1.1

Got the build failure feedback below, this patch is to fix that.

This broke the IA32 Ovmf build for me, with GCC 5.1.1 (Fedora 22):

/home/dwmw2/git/edk2/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c:377:7:
error: ‘PageTables’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
       AsmWriteCr3 (PageTables);
       ^
/home/dwmw2/git/edk2/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c:224:9:
note: ‘PageTables’ was declared here
   UINTN                     PageTables;
         ^
cc1: all warnings being treated as errors

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18190 6f19259b-4bc3-4df7-8a09-765794883524