Create explicit copy constructor for btVector4

[madmiraal]

Currently, when compiling on macOS 15 with Clang 17, I get the following warning:

  src/LinearMath/btVector3.h:1096:2: warning: definition of implicit copy constructor for 'btVector4' is deprecated because it has a user-provided copy assignment operator [-Wdeprecated-copy-with-user-provided-copy]
   1096 |         operator=(const btVector4& v)
        |         ^
  src/LinearMath/btVector3.h:1106:10: note: in implicit copy constructor for 'btVector4' first required here
   1106 |                 return btVector4(_mm_and_ps(mVec128, btvAbsfMask));
        |                        ^

This PR creates an explicit copy constructor for btVector4.