statfs64 ist atm linux specific; wrap statfs around it.
Also cleanup param.h and mount.h including: the assumption in fs.cc
that their presence is a sign of *BSDs is wrong, as they exist in
linux too with different meaning.
Besides presence of mount.h isn't detected in Conky.cmake, so
conditional including mount.h in fs.cc is useless. Is just works
because mount.h and param.h are included in freebsd.h and dragonfly.h
later on. They are unneded in linux.
Just remove conditional including of mount.h and param.h in fs.cc,
conditional wrap statfs64 in proper *BSD header files.
