ada: Formal function default given by expression rejected with -gnatX

The compiler should be allowing formal functions to be defaulted by
an expression when the core extensions are enabled, but it was
requiring all extensions to be enabled.

gcc/ada/ChangeLog:

	* par-ch12.adb (P_Formal_Subprogram_Declaration): Pass True for
	the Is_Core_Extension parameter on call to Error_Msg_GNAT_Extension.