assimp: fix compatibility issue with C++11

src/mesh_loader/assimp.cpp

@@ -32,6 +32,9 @@
  *  POSSIBILITY OF SUCH DAMAGE.
  */
 
+#if __cplusplus < 201103L
+  #define nullptr NULL
+#endif
 #include <hpp/fcl/mesh_loader/assimp.h>
 
 // Assimp >= 5.0 is forcing the use of C++11 keywords. A fix has been submitted https://github.com/assimp/assimp/pull/2758.