[Ada] Fix typo in checks for implementation defined units

[Pierre-Marie de Rodat]

Fix mismatch in iterating over a range for Ada 2012 and referencing an
array for Ada 95. This didn't affect the behaviour, because the
referenced boolean flags in both arrays were the same for the iterated
subrange.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* impunit.adb (Not_Impl_Defined_Unit): Fix typo in iteration
	over Non_Imp_File_Names_12 array.
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