bug: Provide dummy version of bug_get_file_line() when !GENERIC_BUG

Provide the missing dummy bug_get_file_line() implementation when
GENENERIC_BUG isn't selected.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 26dbc7e2 ("bug: Factor out a getter for a bug's file line")
Cc: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>