Fixed building python module with clang 8.0

C++20 has an include file named 'version' that is included in
cstddef with `include <version>'.

On OSX, case insensitive, file python/VERSION is included by mistake.
The problem appears when compiling with clang 8.

I noticed this when building conda package on travis, but it can be
reproduced e.g. using

cd python && make clean && make pip && CC=clang-mp-8.0 python3.7 setup.py build_ext -i

I fixed this putting Plumed.h in a separate directory