Fix clang++ compiler error

When compiling with a recent clang++, I got the following error:

clang++ unittests.cpp -o unittests -lboost_unit_test_framework -std=c++11 -I./include -Wall -Werror -g
In file included from unittests.cpp:50:
./include/cereal/archives/json.hpp:574:18: error: non-const lvalue reference to type 'size_t'
      (aka 'unsigned long') cannot bind to a value of unrelated type 'unsigned long long'
    ar.loadSize( st.size );
[...]

The attached patch fixes the error. I assume this is just a small left-over stray size_t related to commit bfe2289?