Simple-Linked-List - improve compatibility

[vaeng]

Small improve to polish the exercise:

I have one tiny suggestion: Use std::size_t instead of size_t. The latter will work on all > major compilers but technically that's implementation-specific. After including > <cstddef> the standard only guarantees the existence of std::size_t.

Originally posted by @siebenschlaefer in #570 (comment)

[vaeng]

Can you see, why the build is failing @siebenschlaefer ?