Fix output indentation for "info pretty-printers"

Fix output indentation for "info pretty-printers"

I noticed that "info pretty-printers" will indent the "objfile" line
like:

    (top-gdb) info pretty-printer
    global pretty-printers:
      builtin
	mpx_bound128
      objfile /home/tromey/gdb/build/gdb/gdb pretty-printers:
      type_lookup_function

I think the "objfile" line should be "out-dented", following the same
style as the "global" and "progspace" (not shown) lines.

This patch implements this.

gdb/ChangeLog
2018-11-08  Tom Tromey  <tom@tromey.com>

	* python/lib/gdb/command/pretty_printers.py
	(InfoPrettyPrinter.invoke): Don't indent "objfile" heading.