gdb/Makefile.in: Re-sort SUBDIR_UNITTESTS_SRCS/SUBDIR_UNITTESTS_OBS

Note to self: 'o' before 'p'.

gdb/ChangeLog:
2017-04-19  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS, SUBDIR_UNITTESTS_OBS):
	Re-sort in alphabetic order.