ada: Add dependency lines for External_Initialization

ada: Add dependency lines for External_Initialization

When a file included through External_Initialization has been modified,
the unit including it must be recompiled. This patch adds the
generation of dependency lines to the handling of the
External_Initialization aspect, to signal that fact to gnatmake and
other tools that invoke GNAT.

gcc/ada/ChangeLog:
	* lib-writ.ads (Add_Preprocessing_Dependency): Update
	documentation comment.
	* sem_ch3.adb (Apply_External_Initialization): Add call to
	Add_Preprocessing_Dependency.