net: lib: ptp: fix cpp compilation errors

Compilation errors in ptp library when using C++ compiler.
First error is due to 'class' keyword being reserved in C++
and 'class' is used as a variable name in ptp library.
Second error is due to a flexible array member in a struct
is not placed at the end.

Signed-off-by: Tomas Choat <trc@ixys.no>