Sort the output of the leaf reporter

Sort the output of the leaf reporter

It turned out we were not sorting the output of the leaf report.  This
patch fixes that.

	* include/abg-comparison.h (diff_ptrs_type): Define new typedef.
	* src/abg-comparison-priv.h (sort_string_diff_ptr_map): Declare
	new function.
	* src/abg-comparison.cc (sort_string_diff_sptr_map): Update
	comment.
	(sort_string_diff_ptr_map): Define new function.
	* src/abg-leaf-reporter.cc (report_diffs): Sort the diff nodes
	before reporting about them.
	* tests/data/test-diff-suppr/test36-leaf-report-0.txt: Adjust.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>