[hmaptool] Fix dumping

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.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D118004

clang/utils/hmaptool/hmaptool

@@ -44,7 +44,7 @@ class HeaderMap(object):
                         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" % (