[gdb/build] Fix build breaker with -std=c++11

When building with -std=c++11 I run into:
...
gdb/dwarf2/cooked-index.c: In member function \
  ‘void cooked_index::start_writing_index(dwarf2_per_bfd*)’:
gdb/dwarf2/cooked-index.c:469:10: error: lambda capture initializers only \
  available with -std=c++14 or -std=gnu++14 [-Werror]
          ctx = std::move (ctx)] ()
          ^~~
...

Fix this by capturing a copy instead when using -std=c++11:
...
    = gdb::thread_pool::g_thread_pool->post_task ([this, per_bfd, ctx] ()
...

Tested by building with and without -stdc=++11 on x86_64-linux.

Reported-By: Tom Tromey <tom@tromey.com>
Reviewed-by: John Baldwin <jhb@FreeBSD.org>