ShellPkg: UefiShellTftpCommandLib: fix incompatible pointer assignment

ShellPkg: UefiShellTftpCommandLib: fix incompatible pointer assignment

Add the missing EFIAPI calling convention to the CheckPacket() function.
Without it, the gcc build breaks due to the incompatible pointer
assignment in "Mtftp4Token.CheckPacket = CheckPacket".

Cc: Jaben Carsey <jaben.carsey@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18553 6f19259b-4bc3-4df7-8a09-765794883524