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/Common/Assimp.cpp

@@ -1079,7 +1079,7 @@ ASSIMP_API void aiMatrix4DecomposeIntoScalingAxisAnglePosition(
         const C_STRUCT aiMatrix4x4 *mat,
         C_STRUCT aiVector3D *scaling,
         C_STRUCT aiVector3D *axis,
-        float *angle,
+        ai_real *angle,
         C_STRUCT aiVector3D *position) {
     ai_assert(nullptr != mat);
     ai_assert(nullptr != scaling);