libnfs-raw.h: rename export -> exportname to avoid clash with C++ keyword

From http://en.cppreference.com/w/cpp/keyword/export :

  Until C++11:
  "Used to mark a template definition exported, which allows the same
   template to be declared, but not defined, in other translation units."

  Since C++11:
  "The keyword is unused and reserved."

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>