AppPkg: fix webserver build for !Ia32/X64

AppPkg: fix webserver build for !Ia32/X64

The WebServer application is not meant to be Ia32/X64 specific, and would
build for other architectures, if it wasn't for the
  #include <Register/Msr.h>
in WebServer.h. Move that statement to Mtrr.c instead, which is the only
consumer, and is already being filtered out for other architectures.

Cc: Daryl McDaniel <edk2-lists@mc2research.org>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>