bullet: Fix compilation issue with BulletHeightfieldShape

Fixes #1161

panda/src/bullet/bulletHeightfieldShape.cxx

@@ -44,7 +44,7 @@ BulletHeightfieldShape(const PNMImage &image, PN_stdfloat max_height, BulletUpAx
 
   _shape = new btHeightfieldTerrainShape(_num_rows,
                                          _num_cols,
-                                         _data,
+                                         (const void *)_data,
                                          max_height,
                                          up,
                                          true, false);
@@ -102,7 +102,7 @@ BulletHeightfieldShape(Texture *tex, PN_stdfloat max_height, BulletUpAxis up) :
 
   _shape = new btHeightfieldTerrainShape(_num_rows,
                                          _num_cols,
-                                         _data,
+                                         (const void *)_data,
                                          max_height,
                                          up,
                                          true, false);
@@ -127,7 +127,7 @@ BulletHeightfieldShape(const BulletHeightfieldShape &copy) {
 
   _shape = new btHeightfieldTerrainShape(_num_rows,
                                          _num_cols,
-                                         _data,
+                                         (const void *)_data,
                                          _max_height,
                                          _up,
                                          true, false);
@@ -208,7 +208,7 @@ fillin(DatagramIterator &scan, BamReader *manager) {
 
   _shape = new btHeightfieldTerrainShape(_num_rows,
                                          _num_cols,
-                                         _data,
+                                         (const void *)_data,
                                          _max_height,
                                          _up,
                                          true, false);