Use Safe Constants Idioms for ObjFileParser::DEFAULT_MATERIAL.

Reference: "A static Class Member" in https://abseil.io/tips/140

code/AssetLib/Obj/ObjFileParser.h

@@ -42,6 +42,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define OBJ_FILEPARSER_H_INC
 
 #include <assimp/IOStreamBuffer.h>
+#include <assimp/material.h>
 #include <assimp/mesh.h>
 #include <assimp/vector2.h>
 #include <assimp/vector3.h>
@@ -141,7 +142,7 @@ private:
     // because the class contains pointer to allocated memory
 
     /// Default material name
-    static const std::string DEFAULT_MATERIAL;
+    static constexpr char DEFAULT_MATERIAL[] = AI_DEFAULT_MATERIAL_NAME;
     //! Iterator to current position in buffer
     DataArrayIt m_DataIt;
     //! Iterator to end position of buffer