pfunct: Use languages__init() to honour PAHOLE_LANG_EXCLUDE

  $ echo $PAHOLE_LANG_EXCLUDE

  $ pfunct -F dwarf `pahole --running_kernel_vmlinux` > /dev/null
  die__process_class: tag not supported 0x33 (variant_part) at <7719ef8>!
  die__create_new_enumeration: DW_TAG_subprogram (0x2e) @ <0x77370f1> not handled in a rust CU!
  tag__recode_dwarf_type: couldn't find name for function 0x77558a9, abstract_origin=0, specification=0x77370f1
  ^C
  $ export PAHOLE_LANG_EXCLUDE=rust
  $ pfunct -F dwarf `pahole --running_kernel_vmlinux` > /dev/null
  $

Later we need to make it also have --lang_exclude and friends to be just
like pahole, maybe even have all this in a common command line parsing
set of routines, for now its enough to make it obey the env var in the
scripts in tests/.

Cc: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>