Segment: Move definition of MaxChunkSize

This fixes a compile error with clang:

pv/data/segment.cpp.o: In function `unsigned long const& std::min<unsigned long>(unsigned long const&, unsigned long const&)':
/usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/bits/stl_algobase.h:200: undefined reference to `pv::data::Segment::MaxChunkSize'
clang: error: linker command failed with exit code 1 (use -v to see invocation)