dtagnames: cus__load_files() requires a non NULL conf_load

dtagnames: cus__load_files() requires a non NULL conf_load

Since ff7bd7083f36db80 ("core: Allow sizing the loader hash table")
dwarf__load_files(), called by cus__load_files() needs a non-NULL
conf_load argument, add one to avoid a core dump.

With the fix:

  $ dtagnames vmlinux | sort | uniq -c | sort -nr
  8793952 member
  2214363 formal_parameter
  1707497 pointer_type
  1397044 structure_type
  1101747 enumerator
   819003 variable
   671032 subroutine_type
   441683 inlined_subroutine
   432020 lexical_block
   340042 subprogram
   335456 typedef
   264541 array_type
   208998 const_type
   194379 union_type
   132403 enumeration_type
    41631 base_type
    29098 label
     4941 volatile_type
        4 restrict_type
  $

J B reported this for syscse, which led me to do a quick audit to find
this other case.

Reported-by: J B <jb.1234abcd@gmail.com>
Link: https://lore.kernel.org/dwarves/CAJXMFhFbv5MccigKvmQt9MhYwKv2iyigEG3Bhs64BZ=PMWc0WQ@mail.gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>