Fix build when ASSIMP_DOUBLE_PRECISION is on.

Tested:
$ cmake -DASSIMP_DOUBLE_PRECISION=1 -DASSIMP_BUILD_MDL_IMPORTER=0 ..; make

The MDL importer is disabled due to the fact that there is some float-only code
in MDL. This also means bin/assimp and bin/unit would not link successfully with
the above build flags.

code/PostProcessing/ProcessHelper.h

@@ -222,7 +222,7 @@ template <>
 struct MinMaxChooser<aiVertexWeight> {
     void operator()(aiVertexWeight &min, aiVertexWeight &max) {
         MinMaxChooser<unsigned int>()(min.mVertexId, max.mVertexId);
-        MinMaxChooser<float>()(min.mWeight, max.mWeight);
+        MinMaxChooser<ai_real>()(min.mWeight, max.mWeight);
     }
 };