ada: Fix internal error on instance in package body with -gnatn

ada: Fix internal error on instance in package body with -gnatn

This plugs a small loophole in the procedure responsible for attempting to
hide entities that have been previously made public by the semantic analyzer
in package bodies.

gcc/ada/

	* sem_ch7.adb (Hide_Public_Entities): Use the same condition for
	subprogram bodies without specification as for those with one.