fix compiler warnings

tiny_obj_loader.cc

@@ -178,7 +178,7 @@ updateVertex(
     return it->second;
   }
 
-  assert(in_positions.size() > (3*i.v_idx+2));
+  assert(in_positions.size() > (unsigned int) (3*i.v_idx+2));
 
   positions.push_back(in_positions[3*i.v_idx+0]);
   positions.push_back(in_positions[3*i.v_idx+1]);
@@ -721,4 +721,4 @@ std::string LoadObj(
 }
 
 
-};
+}