SPARK emitted spurious checks on Unbounded_String variables that were
not initialized. This patch adds a default initial condition to the type
Unbounded_String so that they disappear.

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

gcc/ada/

	* libgnat/a-strunb.ads, libgnat/a-strunb__shared.ads: Add
	Default_Initial_Condition to Unbounded_String.