Excess float param in TCOD_heightmap_get_minmax

Found a mistake while using DerelictTCOD,  TCOD_heightmap_get_minmax has only two float params.

From heightmap_c.h libtcod 1.5.1:

```
void TCOD_heightmap_get_minmax(const TCOD_heightmap_t *hm, float *min, float *max) {
```