target: Add EXPORT_SYMBOL for existing se_dev_set_*

Now that target_core_backend_configfs.h macros will be using these
se_dev_set attribute functions externally to allow backend drivers
to populate different attributes, go ahead and add EXPORT_SYMBOL()
for the existing default set of 30 device attributes.

Also update target_core_backend.h with proper function prototypes.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>