ada: Fix test for giving hint on ambiguous aggregate

ada: Fix test for giving hint on ambiguous aggregate

In the case the type of an aggregate cannot be determined due to
an ambiguity, caused by the existence of container aggregates,
a hint can be given by GNAT. The test for giving this hint should
be the Ada language version, not the fact that extensions are allowed.
Now fixed.

There is no impact on code generation.

gcc/ada/

	* sem_util.adb (Check_Ambiguous_Aggregate): Fix test.