gdb: Add --with-python-libdir to gdb's --configuration output

Commit:

  commit d13c7322
  Date:   Fri Jan 17 00:10:22 2020 +0000

      gdb: Allow more control over where to find python libraries

Added a new configuration option --with-python-libdir, but failed to
add this option to the output of 'gdb --configuration'.  This commit
fixes this mistake.

gdb/ChangeLog:

	* top.c (print_gdb_configuration): Print --with-python-libdir
	configuration value.