gdb/Makefile: update HFILES_NO_SRCDIR

The HFILES_NO_SRCDIR in our Makefile has gotten a little out of date.
The files: expanded-symbol.h, finish-thread-state.h, and
nat/amd64-linux.h are missing, and there are a few files that are in
the list twice.

This commit adds the missing files and removes the duplicates.

There should be no changes to a built GDB after this commit.

Approved-By: Tom Tromey <tom@tromey.com>