MdeModulePkg: PEI Core: clean up "AprioriFile" handling in FindFileEx()
    
    Clean up two issues around FindFileEx():
    
    - The "AprioriFile" parameter's type differs between the function
      declaration and the function definition. The correct type is
      (EFI_PEI_FILE_HANDLE*).
    
    - "FfsFileHeader" has type (EFI_FFS_FILE_HEADER*); for clarity, we should
      cast it explicitly to EFI_PEI_FILE_HANDLE when assigning it to
      (*AprioriFile).
    
    This is a semantic cleanup, there is no functional change.
    
    Cc: Dandan Bi <dandan.bi@intel.com>
    Cc: Hao A Wu <hao.a.wu@intel.com>
    Cc: Jian J Wang <jian.j.wang@intel.com>
    Cc: Liming Gao <liming.gao@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>