RDMA/mlx5: Fix compilation warning when USER_ACCESS isn't set

RDMA/mlx5: Fix compilation warning when USER_ACCESS isn't set

The cited commit made fs.c always compile, even when
INFINIBAND_USER_ACCESS isn't set. This results in a compilation
warning about an unused object when compiling with W=1 and
USER_ACCESS is unset.

Fix this by defining uverbs_destroy_def_handler() even when
USER_ACCESS isn't set.

Fixes: 36e0d433672f ("RDMA/mlx5: Compile fs.c regardless of INFINIBAND_USER_ACCESS config")
Link: https://patch.msgid.link/r/20250402070944.1022093-1-mbloch@nvidia.com
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>