CorebootModulePkg: Add EFIAPI to OnReadyToBoot to fix gcc compile fail

CorebootModulePkg: Add EFIAPI to OnReadyToBoot to fix gcc compile fail

Make OnReadyToBoot function match the prototype in UefiLib.h. The change
only affects gcc builds because EFIAPI ABI differs from the default ABI
when building with gcc.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Scott Duplichan <scott@notabs.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Maurice Ma <maurice.ma@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17137 6f19259b-4bc3-4df7-8a09-765794883524