[Ada] Fix strange warning when using Ada.Iterator_Interface

[Ada] Fix strange warning when using Ada.Iterator_Interface

The back-end was recently changed to issue more -Wuninitialized warnings
on Out parameters and this has caught a case related to
Ada.Iterator_Interface.: This patchlet simply kills this uninteresting
warning.

2018-10-09  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_disp.adb (Make_Disp_Asynchronous_Select_Spec): Set
	Warnings_Off on the B out parameter.

gcc/testsuite/

	* gnat.dg/warn17.adb: New testcase.

From-SVN: r264977