[Ada] Simplify call to Unique_Defining_Entity on protected entry declarations

[Pierre-Marie de Rodat]

Calling Unique_Defining_Entity on protectected entry declarations is
equivalent to calling a simpler Defining_Entity; use the simpler routine.

Simplification only; semantics unaffected, so no test provided.

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

2018-05-31  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Pragma): Replace call to Unique_Defining_Entity
	with a semantically equivalent call to Defining_Entity.
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