Fix compile error & incorrect push

Fix compile error & incorrect push

I accidentally pushed the wrong version of the patch for commit
7bb43059820c5febb4509b15202a93efde442bc6 (where the review
comments were not fixed), and I did a bad conflict resolution
for ccb1ba62299edce72053dd567b9d384814e11885 leading to a
compile error when libxxhash is available. This fixes both
issues.

gdb/ChangeLog:

2019-10-22  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (struct demangled_name_entry): Add a constructor.
	(free_demangled_name_entry): New function to call the destructor
	for demangled_name_entry.
	(create_demangled_names_hash): Pass free_demangled_name_entry to
	htab_create_alloc.
	(symbol_set_names): Call placement new for demangled_name_entry.
	* utils.c: No longer include xxhash.h here, now that fast_hash
	is inlined in the header.
	* utils.h: Instead, include it here.

Change-Id: If776099d39a65a12733d42efcb859feca1b07a39