[efi] Fix debug directory size

The debug directory size specified in the data directory should cover
only the EFI_IMAGE_DEBUG_DIRECTORY_ENTRY structure, not the whole of
the .debug section.

Reported-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/elf2efi.c

 	debugdir = &(pe_header->nt.OptionalHeader.DataDirectory
 		     [EFI_IMAGE_DIRECTORY_ENTRY_DEBUG]);
 	debugdir->VirtualAddress = debug->hdr.VirtualAddress;
-	debugdir->Size = debug->hdr.Misc.VirtualSize;
+	debugdir->Size = sizeof ( contents->debug );
 
 	return debug;
 }