gallium/auxiliary: Fix pb_bufmgr_slab.c leak

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26223>

src/gallium/auxiliary/pipebuffer/pb_bufmgr_slab.c

@@ -440,6 +440,7 @@ pb_slab_manager_destroy(struct pb_manager *_mgr)
    struct pb_slab_manager *mgr = pb_slab_manager(_mgr);
 
    /* TODO: cleanup all allocated buffers */
+   mtx_destroy(&mgr->mutex);
    FREE(mgr);
 }