[hmaptool] Fix dumping

[smeenai]

Summary

It was complaining about too many values to unpack, since our struct
unpack format string specified six members, but we only had five
variables to unpack to. The sixth value is the max value length, but
it's not used in dumping, so we can ignore it.

[bruno]

LGTM

clang/utils/hmaptool/hmaptool

def frompath(path):
             header_fmt = endian_code + 'HHIIII'
             header_size = struct.calcsize(header_fmt)
             data = f.read(header_size)
             if len(data) != header_size:
                 raise SystemExit("error: %s: truncated headermap header" % (
                         path,))
 
             (version, reserved, strtable_offset, num_entries,
-             num_buckets) = struct.unpack(header_fmt, data)
+             num_buckets, _) = struct.unpack(header_fmt, data)
 
             if version != 1:
                 raise SystemExit("error: %s: unknown headermap version: %r" % (
                         path, version))
             if reserved != 0:
                 raise SystemExit("error: %s: invalid reserved value in header" % (
                         path,))