accel/habanalabs: change unused extern decl of hdev to forward decl of hl_device

Building with clang W=2 has several similar warnings
drivers/accel/habanalabs/common/decoder.c:46:51: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
static void
dec_error_intr_work(struct hl_device *hdev, u32 base_addr, u32 core_id)
                                                  ^
drivers/accel/habanalabs/common/security.h:13:26: note: previous declaration is here
extern struct hl_device *hdev;
                         ^

There is no global definition of hdev, so the extern is not needed.
Searched with
grep -r '^struct' . | grep hl_dev

Change to an forward decl to resolve these issues
drivers/accel/habanalabs/common/mmu/../security.h:133:40: error: ‘struct hl_device’ declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
  133 |         bool (*skip_block_hook)(struct hl_device *hdev,
      |                                        ^~~~~~~~~

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>