ShellPkg: add GUID declaration for FILE_GUID of UEFI Shell app to package

In QuarkPlatformPkg/Library/PlatformBootManagerLib/PlatformBootManager.c,
there is a definition of mUefiShellFileGuid which is a constant reference
to the FILE_GUID as defined in ShellPkg/Application/Shell/Shell.inf.

To prevent the need for duplicating it to other modules, promote it to
a proper global GUID, and add it to the ShellPkg.dec package declaration.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>