[gdb/build, c++20] Fix more deprecated implicit capture of this

[gdb/build, c++20] Fix more deprecated implicit capture of this

When building gdb with -std=c++20 I run into:
...
gdb/dwarf2/cooked-index.c: In lambda function:
gdb/dwarf2/cooked-index.c:471:47: error: implicit capture of ‘this’ via \
  ‘[=]’ is deprecated in C++20 [-Werror=deprecated]
  471 |   gdb::thread_pool::g_thread_pool->post_task ([=] ()
      |                                               ^
gdb/dwarf2/cooked-index.c:471:47: note: add explicit ‘this’ or ‘*this’ capture
...

Fix this and two more spots by removing the capture default, and explicitly
listing all captures.

Tested on x86_64-linux.