module: fix building with sysfs disabled

Sysfs support might be disabled so we need to guard the code that
instantiates "compression" attribute with an #ifdef.

Fixes: b1ae6dc4

 ("module: add in-kernel support for decompressing")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>