fix compiler warnings

I get these compiler warnings when using tinyobjloader in my own projects so I thought of fixing them.
```
tiny_obj_loader.h:100:2: warning: extra ';' [-Wpedantic]
 };
  ^
```
```
tiny_obj_loader.cc:181:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   assert(in_positions.size() > (3*i.v_idx+2));
```
```
tiny_obj_loader.cc:724:2: warning: extra ';' [-Wpedantic]
 };
  ^
```