readelf: Print actual file for decl_file and call_file attributes.

readelf: Print actual file for decl_file and call_file attributes.

When we see a DW_AT_decl_file or DW_AT_call_file attribute print the
actual file name. The current interface gives us a full (absolute) patch,
but we only want to show the file name for now to not clutter the output
too much. This helps a lot when trying to determine where something was
declared if you are just looking at the DIE tree. Otherwise you'll have
to cross match the number by hand with the corresponding line table entry.

Signed-off-by: Mark Wielaard <mark@klomp.org>