[krbdev.mit.edu #5602] Fix warnings in 1.6.2 serialize.c

[Public Submitter via RT]

In 1.6.2, in file src/lib/krb5/krb/serialize.c,  function 
krb5_register_serializer, all three calls to memcpy() are receiving a 
const pointer as their first parameter.  GCC complains with a 
warning.  Other compilers (TANDEM Nonstop) drop dead with an error.

Lines: 73, 76, and 86 from the stock 1.6.2 release