[rmodels] Fix memory leak in UnloadModel()

[LuisR385]

Description

There is a memory leak in UnloadModel() when loading and unloading 3D models with bones (skinned models).
LoadModel() allocates memory for model.currentPose and model.boneMatrices, but UnloadModel() does not free them.

Scope of leak

Tested with a GLB model containing 43 bones on raylib 6.0 (Windows / Visual Studio).
Leak sizes reported by Visual Studio exactly match the bone allocations:

• 43 * sizeof(Transform) = 1720 bytes
• 43 * sizeof(Matrix) = 2752 bytes

Solution

Added RL_FREE(model.currentPose); and RL_FREE(model.boneMatrices); inside UnloadModel() right before the TRACELOG.

The RL_FREE macro safely checks for NULL, so this will not cause any issues or crashes for non-animated models (models without bones).

[raysan5]

@LuisR385 thanks for the review!