Fix v140 compile errror

code/AssetLib/Obj/ObjFileParser.cpp

@@ -55,7 +55,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 namespace Assimp {
 
-constexpr char ObjFileParser::DEFAULT_MATERIAL[];
+constexpr const char ObjFileParser::DEFAULT_MATERIAL[];
 
 ObjFileParser::ObjFileParser() :
         m_DataIt(),

code/AssetLib/Obj/ObjFileParser.h

@@ -141,7 +141,7 @@ private:
     // because the class contains pointer to allocated memory
 
     /// Default material name
-    static constexpr char DEFAULT_MATERIAL[] = AI_DEFAULT_MATERIAL_NAME;
+    static constexpr const char DEFAULT_MATERIAL[] = AI_DEFAULT_MATERIAL_NAME;
     //! Iterator to current position in buffer
     DataArrayIt m_DataIt;
     //! Iterator to end position of buffer