MdeModulePkg/BootMaintManagerUiLib: Declare EFIAPI for ChooseFile handler

MdeModulePkg/BootMaintManagerUiLib: Declare EFIAPI for ChooseFile handler

The BootMaintenanceManagerUiLib use ChooseFile() from FileExplorerLib
to select files. And the third parameter in ChooseFile() is CHOOSE_HANDLER,
per the definition of CHOOSE_HANDLER, it must use EFIAPI as the calling
convention. But the calling convention was not specified for following
handlers: CreateBootOptionFromFile, CreateDriverOptionFromFile,
BootFromFile. Now specifies the calling convention for those functions.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Gary Lin <glin@suse.com>