assimp: fix bfin compile

Use proper 64-bit constant for CONVERT_FBX_TIME(time) conversion.

Fixes ([1]):

  code/FBXConverter.cpp:2025: error: integer constant is too large for 'long' type
  code/FBXConverter.cpp:2026: error: integer constant is too large for 'long' type
  code/FBXConverter.cpp:2794: error: integer constant is too large for 'long' type
  code/FBXConverter.cpp:2868: error: integer constant is too large for 'long' type
  code/FBXConverter.cpp:2878: error: integer constant is too large for 'long' type
  code/FBXConverter.cpp:2888: error: integer constant is too large for 'long' type

[1] http://autobuild.buildroot.net/results/885/8853b192d16ca7ef769c5352a2df0540a7a2a4fd



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>