gdb/ctf: add scoped_time_it in elfctf_build_psymtabs

This will be useful to determine the impact on the startup time getting
rid of partial symtabs has.

I use it like so:

    $ ./gdb -q -nx --data-directory=data-directory -iex "maint set per-command time on" /home/simark/src/linux/vmlinux.unstripped -batch
    ...
    Time for "elfctf_build_psymtabs": wall 0.381, user 0.357, sys 0.015, user+sys 0.372, 97.6 % CPU

Change-Id: I021319212f27eee0bf0f6c230f4e7cdd9c3602c1
Approved-By: Tom Tromey <tom@tromey.com>